Machine Learning Foundations at Taiwan AI Academy

Machine Learning Foundations
2018/02/05-06 @ Taiwan AI Academy
Albert Y. C. Chen, Ph.D.
Vice President, R&D
Viscovery

Albert Y. C. Chen, Ph.D.
陳彥呈博⼠士
albert@viscovery.com
http://www.linkedin.com/in/aycchen
http://slideshare.net/albertycchen
• Experience
2017-present: Vice President of R&D @ Viscovery
2015-2017: Chief Scientist @ Viscovery
2015-2015: Principal Scientist @ Nervve Technologies
2013-2014 Senior Scientist @ Tandent Vision Science
2011-2012 @ GE Global Research, Computer Vision Lab
• Education
Ph.D. in Computer Science, SUNY-Buffalo
M.S. in Computer Science, NTNU
B.S. in Computer Science, NTHU

When something is important enough,
you do it even if the odds are not in your favor.
Elon Musk
Falcon 9
takeoff
Falcon 9
decelerate
Falcon 9
vertical
touchdown

What is “Machine Learning”?
• Machine Learning (ML):
• Human Learning:
• Manual Programming:
rules

• Deterministic problems: repeat 1B
times, still get the same answer,
• problems lacking data,
• problems with easily separable data.
Manual Programming vs Machine Learning
• Data with noise,
• data of high dimension,
• data of large volume,
• data that changes over time.
When to manual program?
When to use machine learning?

• Important concepts (lessons learned) from
classical machine learning are still very
important, from dimensionality, sampling,
distance measures, error metrics, and
generalization issues.
• Understand how things work, why things worked
in the past, and why previously unattainable
problems are solved by Deep Learning.
Deep Learning, directly?

We present you,
a simple & usable map for ML!
Dimension
Reduction
Clustering
Regression Classiﬁcation
continuous
(predicting a quantity)
discrete
(predicting a category)
supervisedunsupervised

• Before we start, we need to estimate data
distribution and develop sampling strategies,
• figure out how to measure/quantify data, or, in
other words, represent them as features,
• figure out how to split data to training and
validation set.
• After we learn a model, we need to measure the
fit, or the error on validation set.
• Finally, how do we evaluate how well our trained
model generalize.
Steps for Supervised Learning

Sampling & Distributions
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The importance of good sampling & distribution estimation.
Population with attribute
modeled by functionf : X ! Y
X Y
Learn from D =
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sample
x 2 X, y 2 Y
{(x1, y1), (x2, y2), ..., (xN , yN )}
f0
incorrectly predicts that
everyone else “smiles crazily”
f0

• The chances of getting a "perfect" sample of the
population at ﬁrst try is very very small. When
the population is huge, this problem worsens.
• Noise during the measurement process adds
additional uncertainties.
• As a result, it is natural to try multiple times, and
formulate the problem in a probabilistic way.
Sampling & Distributions

• Joint probability of X taking
the value xi and Y taking the
value yi :
• Marginalizing: probability
that X takes the value xi
irrespective of Y:
Probability Theory
yj nij
xi
} rj
}
ci
p(X = xi, Y = yi) =
nij
N
p(X = xi) =
ci
N
, where ci =
X
j
nij

• Conditional Probability: the
fraction of instances where Y
= yj given that X = xi.
• Product Rule:
Probability Theory
yj nij
xi
} rj
}
ci
p(Y = yj|X = xi) =
nij
ci
p(X = xi, Y = yj) =
nij
N
=
nij
ci
·
ci
N
= p(Y = yj|X = xi)p(X = xi)

• Bayes' Rule plays a central
role in pattern recognition
and machine learning.
• From the product rule,
together with the symmetric
property
we get:
Bayes' Rule
yj nij
xi
} rj
}
ci
p(X, Y ) = p(Y, X)
p(Y |X) =
p(X|Y )p(Y )
p(X)
, where p(X) =
X
Y
p(X|Y )p(Y )

• p(Y = a) = 1/4, p(Y = b) = 3/4
• p(X = blue | Y = a) = 3/5
• p(X = green | Y = a) = 2/5
When we randomly draw a ball that is blue, the
probability that it comes from Y=a is?
Bayes' Rule Example 1
Y=a Y=b
p(Y = a|X = blue) =
p(X = blue|Y = a)p(Y = a)
p(X = blue)
=
p(X = blue|Y = a)p(Y = a)
(p(X = blue|Y = a)p(Y = a) + (p(X = blue|Y = b)p(Y = b)
=
3
5 · 1
4
3
5 · 1
4 + 2
5 · 3
4
=
3
20
3
20 + 6
20
=
3
20
9
20
=
1
3

• Monty Hall problem
• Prize behind one of the three
doors. After choosing door 1,
the host opens empty door 3
and asks if you want to switch
your choice. Should you switch?
Bayes' Rule Example 2
1 2 3
?
Behind
door 1
Behind
door 2
Behind
door 3
Result if staying
at door 1
Result if switching to
the door offered
Car Goat Goat Wins car Wins goat
Goat Car Goat Wins goat Wins car
Goat Goat Car Wins goat Wins car

When we measure the wrong
features, we’ll need very
complicated classiﬁers, and
the results are still not ideal.
Features
baseball tennis ball
vs
There’s always “exceptions”
that would ruin our perfect
assumptions yellow
baseball?
we learn the best features from data with deep learning.

• More features ≠ better: number of features*N,
feature space grows by ^N, the number of samples
needed for ML grows proportionally as well.
The curse of dimensionality

• Most of the volume of an n-D sphere is
concentrated in a thin shell near the surface!!!
• nD sphere of , the volume of sphere
between and is:
The curse of dimensionality
r = 1
r = 1 ✏ r = 1 1 (1 ✏)D

• The curse of dimensionality not just effects the
feature space, but also input, output, and others.
• Much more challenging to train a good n-class
classifier, e.g., face recognition, 1-to-1
verification vs 1-to-n identification.
• Much more issues arise from using a general
purpose 1M-class classifier vs problem
specific 1k-class classifier.
High-dim. issue is prevalent

Recognition
Accuracy:
• 1 to 1: 99%+
• 1 to 100: 90%
• 1 to 10,000:
50%-70%.
• 1 to 1M: 30%.
LFW dataset, common FN↑, FP↓
Prevalent high-dim issue, eg.1
• 1-to-N face identiﬁcation, in the wild!

Prevalent high-dim issue, eg.2
• Smart photo album, with Google Cloud Vision
Distance between
histograms of 1M bins
is very close to 0 for
most of the time.

• Real data will often be conﬁned to a region of
the space having lower effective dimensionality.
• Data will typically exhibit some smoothness
properties (at least locally).
Living with high dimensions
E.g., Low-dimensional
“manifold” of faces,
embedded within a
high-dim space.
Keywords:
• dimension reduction,
• learned features,
• manifold learning.

• k-fold cross validation
Splitting data
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Repurposing the smily faces
ﬁgures to represent the set of
annotated data.
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Randomly split into k groups

• Minimizing the misclassiﬁcation rate
• Minimizing the expected loss
• The reject option
Decision Theory

• Decision boundary, or simply, in 1D, a threshold,
s.t. anything larger than the threshold are
classiﬁed as a class, and smaller than the
threshold as another class.
Decision Boundary

• Different metrics & names used in different fields
for measuring ML performance; however, the
common cornerstones are:
• True positive (TP): sample is an apple,
classified as an apple.
• False positive (FP): sample is not an apple, but
classified as an apple.
• True negative (TN): sample is not an apple,
classified as not an apple.
• False negative (FN): sample is an apple, but
misclassified as "not an apple.
True/False, Positive/Negative

• Precision:  
 
Classifier identified (TP+FP)
apples, only TP are apples.
(aka positive predictive value.)
• Recall: 
 
Total (TP+FN) apples,
classifier identified TP.  
(aka, hit rate, sensitivity, true
positive rate)
Precision vs Recall
TP
TP + FP
TP
TP + FN

• F-measure:  
 
harmonic mean of precision and recall. F-
measure is criticized outside Information
Retrieval ﬁeld for neglecting the true negative.
• Accuracy (ACC):  
 
a weighted arithmetic mean of precision and
inverse precision, as well as the weighted
arithmetic mean of recall and inverse recall.
A single balanced metric?
TP + TN
TP + TN + FP + FN
2 ·
precision · recall
precision + recall

Multi-objective Optimization
e.g., micro air vehicle wing design

• Different types of errors are weighted differently;
e.g., medical examinations, minimize false
negative but can tolerate false positive.
• Reformulate objectives from maximizing
probability to minimizing weighted loss
functions.
• The reject option: refrain from making decisions
on difﬁcult cases (e.g., for samples within a
certain region inside the decision boundary.)
Minimizing the expected loss

• Minimizing Training and Validation Error, v.s.
minimizing Testing Error.
• Memorizing every “practice exam” question ≠
doing well on new questions. Avoid overﬁtting.
Generalization
E.g., training a classiﬁer
that recognizes trees

• Bias:
• Difference between the expected (or
averaged) prediction of our model and the
correct value.
• Error due to inaccurate assumptions/
simpliﬁcations.
• Variance:
• Amount that the estimate of the target function
will change if different training data was used.
Generalization Error

Bias/variance trade-off
Scott Fortmann-Roe

• Model is too simple to represent all the relevant
class characteristics.
• High bias (few degrees of freedom, DoF) and
low variance.
• High training error and high test error.
Underﬁtting

• Model is too complex and ﬁts
irrelevant noise in the data
• Low bias, high variance
• Low training error, high test error
Overﬁtting

Error (mean square error, MSE)  
= noise2 + bias2 + variance
Bias-Variance Trade-off
unavoidable
error
error due to incorrect
assumptions made
about the data
error due to variance
of training samples

Model Complexity
Slide credit: D. Hoiem

Training Sample vs Model Complexity

Effect of Training Sample Size

• Models: describe relationship between variables
• Deterministic models: hypothesize exact
relationships, OK when noise is negligible
• Probabilistic models: deterministic part +
random error. For example:
• Regression models: one dependent
variable + one or more numerical or
categorical independent (explanatory)
variable.
• Correlation models: multiple independent
variables.
How do we learn models?

Generative vs Discriminative Models
Discriminative Model:
directly learn the data
boundary
Generative Model:
represent the data
and boundary

• Learn to directly predict labels from the data
• Often uses simpler boundaries (e.g., linear) for
hopes of better generalization.
• Often easier to predict a label from the data than
to model the data.
• E.g.,
• Logistic Regression
• Support Vector Machines
• Max Entropy Markov Model
• Conditional Random Fields
Discriminative Models

• Represent both the data and the boundary.
• Often use conditional independence and priors.
• Modeling data is challenging; need to make and
verify assumptions about data distribution
• Modeling data aids prediction & generalization.
• E.g.,
• Naive Bayes
• Gaussian Mixture Model (GMM)
• Hidden Markov Model
• Generative Adversarial Networks (GAN)
Generative Models

• Bernoulli Distribution
• Uniform Distribution
• Binomial Distribution
• Normal Distribution
• Poisson Distribution
• Exponential Distribution
Distributions

Dimension Reduction
Machine Learning Roadmap
Dimension
Reduction
Clustering
continuous
discrete

• Goal: try to ﬁnd a more compact
representation of the data
• Assume that the high
dimensional data actually
reside in an inherent low-
dimensional space.
• Additional dimensions are 
just random noise
• Goal is to recover these
inherent dimensions and
discard noise.
Unsupervised Dimension Reduction

• Create a basis where
the axes represent the
dimensions of variance,
from high to low.
• Finds correlations in
data dimensions to
product best possible
lower-dimensional
representation based
on linear projections.
Principal Component Analysis (PCA)

PCA algorithm, conceptual steps
• Find a line s.t. when data is projected onto the
line, it has the maximum variance.

• Find new line orthogonal to the ﬁrst that has the
maximum projected variance.

• Repeated until d lines. The projected position of
a point on these lines gives the coordinates in
the m-dimensional reduced space.
• Computing these set of lines is achieved by
eigen-decomposition of the covariance matrix.

• Given n data points: x1, ..., xn
• Consider a linear projection speciﬁed by v
• The projection of x onto v is
• The variance of the projected data is
• The 1st Principal Component maximizes the
variance subject to the constraint
PCA, maximizing variance
z = vT
x
var(z) = var(vT
xv) = vT
var(x)v = vT
Sv

• Maximize , subject to
• Lagrange:
• is the eigen-vector of S with eigen-value
• Sample variance of the projected data
• The eigen-values equals the amount of variance
captured by each eigen-vector
PCA, maximizing variance
vT
Sv vT
v = 1
vT
Sv (vT
v 1)
d
dv
= 0 ! Sv = v
v
vT
Sv = vT
v =

• View PCA as minimizing the reconstruction error
of using a low-dimensional approximation of the
original data:
Alternative view of PCA
x1
⇡ x0 + z1
u x2
⇡ x0 + z2
u

• Calculate the covariance matrix of the data S
• Calculate the eigen-vectors/eigen-values of S
• Rank the eigen-values in decreasing order
• Select eigen-vectors that retain a ﬁxed % of the
variance, e.g., 80%, s.t.,
Dimension Reduction using PCA
Pd
i=1 i
P
i i
80%

PCA example: Eigenfaces
Mean face
Basis of variance (eigenvectors)
M. Turk; A. Pentland (1991). "Face recognition using eigenfaces".
Proc. IEEE Conference on Computer Vision and Pattern Recognition. pp. 586–591.

The ATT face database (formerly the ORL
database), 10 pictures of 40 subjects each

• Covariance of the image data is big. Finding
eigenvector of large matrices is slow.
• Singular Value Decomposition (SVD) can be
used to compute principal components.
• SVD steps:
• Create centered data matrix X
• Solve: X = USVT
• Columns of V are the eigenvectors of
sorted from largest to smallest eigenvalues.
PCA, scaling up
⌃

• Useful preprocessing for easing the "curse of
dimensionality" problem.
• Reduced dimension: simpler hypothesis
space
• Smaller VC dimension: less overﬁtting
• PCA can also be seen as noise reduction
• Fails when data consists of multiple separate
clusters
PCA discussion

• Also named Fisher Discriminant Analysis
• It can be viewed as
• a dimension reduction method,
• a generative classiﬁer p(x|y), Gaussian with
distinct for each class but shared .
Linear Discriminant Analysis (LDA)
µ ⌃
classes mixed better separation

• Find a project direction so that the separation
between classes is maximized.
• Objective 1: maximize the distance between the
projected means of different classes
LDA Objectives
m1 =
1
N1
X
x2C1
x m2 =
1
N2
X
x2C2
x
original means:
projected means:
m0
1 =
1
N1
X
x2C1
wT
x m0
2 =
1
N2
X
x2C2
wT
x

• Objective 2: minimize scatter (variance within
class)
LDA Objectives
s2
i =
X
x2Ci
(wT
x m0
i)2Total within class scatter
for projected class i:
Total within class scatter: s2
1 + s2
2

• There are a number of different ways to combine
the two objectives.
• LDA seeks to optimize the following objective:
LDA Objective

LDA for two classes
w = S 1
w (m1 m2)

• Objective remains the same, with slightly
different deﬁnition for between-class scatter:
• Solution: k-1 eigenvectors of
LDA for Multi-Classes
J(w) =
wT
SBw
wTSww
SB =
1
k
kX
i=1
(mi m)(mi m)T
S 1
w SB

• Data often lies on
or near a nonlinear
low-dimensional
curve.
• We call such a
low-d structure
manifolds
• Algorithms include:
ICA, LLE, Isomap.
Nonlinear Dimension Reduction
swiss roll data

• A non-linear method for dimensionality reduction
• Preserves the global, nonlinear geometry of the
data by preserving the geodesic distances.
• Geodesic: shortest route between two points on
the surface of a manifold.
ISOMAP: Isometric Feature Mapping

1. Approximate the geodesic distance between
every pair of points in the data.
• The manifold is locally linear
• Euclidean distance works well for points that
are close enough.
• For points that are far apart, their geodesic
distance can be approximated by summing
up local Euclidean distances.
2. Find a Euclidean mapping of the data that
preserves the geodesic distance.
ISOMAP algorithm

• Construct a graph by:
• Connecting i and j if:
• d(i,j) < (if computing -isomap), or
• i is one of j's k nearest neighbors (k-isomap)
• Set the edge weight equal d(i,j) - Euclidean
distance
• Compute the Geodesic distance between any
two points as the shortest path distance.
Geodesic Distance
" "

• We can use Multi-Dimensional Scaling (MDS), a
class of statistical techniques that:
• Given:
• n x n matrix of dissimilarities between n
objects
• Outputs:
• a coordinate conﬁguration of the data in low-d
space Rd whose Euclidean distances closely
match given dissimilarities.
Compute low-dimensional mapping

Regression
Dimension
Reduction
Clustering
continuous
discrete

• Unit-less, normalized between [-1, 1]
Pearson’s Correlation Coefﬁcient
Y
X
Y
X
Y
X
Y
X
Y
X
r = -1 r = -.6 r = 0
r = +.3r = +1
Y
X
r = 0
Figures modified from: Statistics for Managers Using Microsoft® Excel 4th Edition, 2004 Prentice-Hall
r =
cov(x, y)
p
var(x)
p
var(y)

Linear Correlations
Y
X
Y
X
Linear relationships
Y
Y
X
X
Curvilinear relationships
Y
X
Y
X
Strong relationships
Y
Y
X
X
Weak relationships
Y
X
No relationship
Y
X

• In correlation, two variables are treated as
independent.
• In regression, one variable (x) is independent,
while the other (y) is dependent.
• Goal: if you know something about x, this would
help you predict something about y.
Regression

• Expected value at a
given level of x:
• Predicted value for a
new x:
Simple Linear Regression
y
x
random error that
follows a normal distribution
with 0 mean and variance
"
2
ﬁxed exactly
on the line
y = w0 + w1x
y0
= w0 + w1x + "
w0
w0/w1

Multiple Linear Regression
y(x, w) = w0 + w1x1 + · · · + wDxD
w0, ..., wD
xi
• Linear function of parameters , also a
linear function of the input variables , has very
restricted modeling power (can't even ﬁt curves).
• Assumes that:
• The relationship between X and Y is linear.
• Y is distributed normally at each value of X.
• The variance of Y at each value of X is the
same.
• The observations are independent.

• Before going further, let’s take a look at
polynomial line ﬁtting (polynomial regression.)
Linear Regression
Given N=10 blue dots, try to ﬁnd the function
that is used for generating the data points.
sin(2⇡x)

• Polynomial line ﬁtting:
• M is the order of the polynomial
• linear function of the coefﬁcients
• nonlinear function of
• Objective: minimize the error between the
predictions and the target value of
Polynomial Regression
x
w
y(xn, w) tn xn
ERMS =
p
2E(w⇤)/Nor, the root-mean-square error
E(w) =
1
2
NX
n=1
{y(xn, w) tn}
2
y(x, w) = w0 + w1x + w2x2
+ · · · + wM xM
+ "

Polynomial regression w. var. M

• There's only 10 data points, i.e., 9 degrees of
freedom; we can get 0 training error when M=9.
• Food for thought: make sure your deep neural
network's is not just "memorizing the training
data when its M >> data's DoF.
Polynomial regression w. var. M

• With M=9, but N=15 (left) and N=100, the over-
ﬁtting problem is greatly reduced.
• ML is all about balancing M and N. One rough
heuristic is that N should be 5x-10x of M (model
complexity, not necessarily the number of param.)
What happens with more data?

• Regularization: used for controlling over-ﬁtting.
• E.g., discourage coefﬁcients from reaching
large values: 
 
 
 
where
Regularization
˜E(w) =
1
2
NX
n=1
{y(xn, w) tn}
2
+
2
||w||2
||w||2
= wT
w = w2
0 + w2
1 + · · · + w2
M

• Extending linear regression to linear
combinations of ﬁxed nonlinear functions: 
 
 
 
where
• Basis functions: act as "features" in ML.
• Linear basis function:
• Polynomial basis function:
• Gaussian basis function
• Sigmoid basis function
Linear Models for Regression
y(x, w) =
M 1X
j=0
wj (x)
w = (w0, . . . , wM 1)T
, = ( 0, . . . , M 1)T
{ j(x)}
j(x) = xj
j(x) = xj

• Global functions of
the input variable,
s.t. changes in one
region of input
space affect all
other regions.
Polynomial Basis Functions
j(x) = xj

• Local functions, a
small change in x
only affect nearby
basis functions.
• and control
the location and
scale (width).
Gaussian Basis Functions
j(x) = exp
⇢
(x µj)2
2s2
µj s

• Local functions, a
small change in x
only affect nearby
basis functions.
• and control
the location and
scale (slope).
Sigmoidal Basis Functions
µj s
j(x) =
✓
x µj
s
◆
(a) =
1
1 + exp( a)
where

• Adding a regularization term to an error function:
• One of simplest forms of regularizer is sum-of-
squares of the weight vector elements:
• This type of weight decay regularizer (in ML),
a.k.a., parameter shrinkage (in statistics)
encourages weight values to decay towards
zero, unless supported by the data.
Regularized Least Squares
EW (w) =
1
2
wT
w
ED(w) + EW (w)

• A more general regularizer in the form of:
• q=2 is the quadratic regularizer (last page).
• q=1 is known as lasso in statistics.
Regularized Least Squares
1
2
NX
n=1
tn wT
(xn)
2
+
2
MX
j=1
|wj|q
sum of squared error generalized regularizer,

• LASSO: least absolute shrinkage and selection
operator
• When is sufﬁciently large, some of the
coefﬁcients are driven to zero, leading to a
sparse model
LASSO
wj

• Large values of : small variance but large bias
• Small values of : large variance, small bias
The Bias-Variance Tradeoff

Clustering
Dimension
Reduction
Clustering
continuous
discrete

• Group together similar points and represent
them with a single token.
• Issues:
• How do we deﬁne two points/images/patches
being "similar"?
• How do we compute an overall grouping from
pairwise similarity?
Clustering

• Grouping pixels of similar appearance and
spatial proximity together; there's so many ways
to do it, yet none are perfect.
Clustering Example

• Summarizing Data
• Look at large amounts of data
• Patch-based compression or denoising
• Represent a large continuous vector with the
cluster number
• Counting
• Histograms of texture, color, SIFT vectors
• Segmentation
• Separate the image into different regions
• Prediction
• Images in the same cluster may have the same
labels
Why do we cluster?

• K-means
• Iteratively re-assign points to the nearest cluster
center
• Gaussian Mixture Model (GMM) Clustering
• Mean-shift clustering
• Estimate modes of pdf
• Hierarchical clustering
• Start with each point as its own cluster and
iteratively merge the closest clusters
• Spectral clustering
• Split the nodes in a graph based on assigned
links with similarity weights
How do we cluster?

• Goal: cluster to minimize variance in data given
clusters while preserving information.
Clustering for Summarization
c⇤
, ⇤
= argmin
c,
1
N
NX
j=0
KX
i=0
i,j(ci xj)2
cluster center
data
Whether is assigned toxj ci

• Euclidean Distance:
• Cosine similarity:
How do we measure similarity?
✓ = arccos
✓
xy
|x||y|
◆
x
y
||y x|| =
p
(y x) · (y x)
distance(x, y) =
p
(y1 x1)2 + (y2 x2)2 + · · · + (yn xn)2
=
v
u
u
t
nX
i=1
(yi xi)2
x · y = ||x||2 ||y||2 cos ✓
similarity(x, y) = cos(✓) =
x · y
||x||2 ||y||2

• Compare distance of closest (NN1) and second
closest (NN2) feature vector neighbor.
• If NN1≈NN2, ratio NN1/NN2 will be ≈1 →
matches too close.
• As NN1 << NN2, ratio NN1/NN2 tends to 0.
• Sorting by this ratio puts matches in order of
conﬁdence.
Nearest Neighbor Distance Ratio

• How to threshold the nearest neighbor ratio?
Nearest Neighbor Distance Ratio
Lowe IJCV
2004 on
40,000
points.
Threshold
depends on
data and
speciﬁc
applications

1. Randomly select k initial cluster centers
2. Assign each point to nearest center 
 
3. Update cluster centers as the mean of the points 
 
4. repeat 2-3 until no points are re-assigned.
k-means clustering
t
= argmin
1
N
NX
j=1
KX
i=1
i,j ct 1
i xj
2
ct
= argmin
c
1
N
NX
j=1
KX
i=1
t
i,j (ci xj)
2

• Initialization
• Randomly select K points as initial cluster
center
• Greedily choose K points to minimize residual
• Distance measures
• Euclidean or others?
• Optimization
• Will converge to local minimum
• May want to use the best out of multiple trials
k-means: design choices

• Cluster on one set, use another (reserved) set to
test K.
• Minimum Description Length (MDL) principal for
model comparison.
• Minimize Schwarz Criterion, a.k.a. Bayes
Information Criteria (BIC)
• (When building dictionaries, more clusters
typically work better.)
How to choose k

• Generative
• How well are points reconstructed from the
cluster?
• Discriminative
• How well do the clusters correspond to labels
(purity)
How to evaluate clusters?

• Pros
• Finds cluster center that minimize conditional
variance (good representation of data)
• simple and fast
• easy to implement
k-means pros & cons

• Cons
• Need to choose K
• Sensitive to outliers
• Prone to local minima
• All clusters have the same parameters
• Can be slow. Each iteration is O(KNd) for N d-
dimensional points
k-means pros & cons

• Clusters are spherical
• Clusters are well separated
• Clusters are of similar volumes
• Clusters have similar number of points
k-means works if

• Hard assignments, or probabilistic assignments?
• Case against hard assignments:
• Clusters may overlap
• Clusters may be wider than others
• Can use a probabilistic model,
• Challenge: need to estimate model
parameters without labeled Ys.
GMM Clustering
P(X|Y )P(Y )

• Assume m-dimensional data points
• still multinomial, with k classes
• are k
multivariate Gaussians
Gaussian Mixture Models
P(Y )
P(X|Y = i), i = 1, · · · , k
P(X = x|Y = i)
=
1
p
(2⇡)m|⌃i|
exp
✓
1
2
(x µi)T
⌃ 1
(x µi)
◆
mean (m-dim vector)
variance (m*m matrix)
determinant of matrix

• Estimating parameters (when given data label Y)
• Solve optimization problem:
• MLE has closed form solution:
• i.e., solve
• Estimating parameters (without ), solve:
: all model param including mean, variance, etc.
Maximum Likelihood Estimation (MLE)
P(X = x|Y = i) = 1p
(2⇡)m|⌃i|
exp 1
2 (x µi)T
⌃ 1
(x µi)
µML = 1
n
Pn
i=1 xi
⌃ML = 1
n
Pn
i=1(xi
µML)(xi
µML)T
argmax✓
Q
j P(yj
, xj
; ✓)
yj
argmax✓
Q
j P(xj
, ✓) = argmax
Q
j
Pk
i=1 P(yj
= i, xj
; ✓)
✓

• Maximize marginal likelihood
• Almost always a hard problem
• Usually no closed form solution
• Even when is convex,
generally isn't
• For all but the simplest , we will have
to do gradient ascent, in a big messy space
with lots of local optimum.
Solving MLE for GMM Clustering
argmax✓
Q
j P(xj
, ✓) = argmax
Q
j
Pk
i=1 P(yj
= i, xj
; ✓)
P(X, Y ; ✓) P(X; ✓)
P(X; ✓)

• Simple example: GMM with
1D data, k=2 Gaussians,
variance=1, distribution over
classes is uniform, only need
to estimate , .
µ1 µ2
nY
j=1
kX
i=1
P(X = xj
, Y = i) /
nY
j=1
kX
i=1
exp
✓
1
2 2
(xj
µi)2
◆
• Skipping the derivations.... still need to
differentiate and solve for , and P(Y=1) for
i=1...k. There are still no closed form solution,
gradient is complex with lots of local optimum.
µi ⌃i

• Expectation Maximization
• Objective:
• Data:
• E-step: For all examples j and values i for y,
compute:
• M-step: re-estimate the parameters with
weighted MLE estimates, set:
argmax
✓
Y
j
kX
i=1
P(yj
= i, xj
|✓) =
X
j
log
kX
i=1
P(yj
= i, xj
|✓)
{xj
|j = 1 . . . n}
P(yj
= i|xj
, ✓)
✓ = argmax✓
P
j
Pk
i=1 P(yj
= i|xj
, ✓) log P(yj
= i, xj
|✓)

EM for GMM MLE example
1 2 3
4 5 6

• EM after 20 iterations

• GMM for some bio assay data

• GMM for some bio
assay data, ﬁtted
separately for three
diffrent compounds.

• GMM with hard assignments and unit variance,
EM is equivalent to k-means clustering
algorithm!!!
• EM, like k-NN, uses coordinate ascent, and can
get stuck in local optimum.
EM for GMM Clustering, notes

• mean-shift seeks modes of a given set of points
1. Choose kernel and bandwidth
2. For each point:
1. center a window on that point
2. compute the mean of the data in the
search window
3. center the search window at the new
mean location, repeat 2,3 until converge.
3. Assign points that lead to nearby modes to
the same cluster.
Mean-Shift Clustering

• Try to ﬁnd modes of a non-parametric density
Mean-shift algorithm
Color
space
Color
space
clusters

• Attraction basin: the region for which all
trajectories lead to the same mode.
• Cluster: all data points in the attraction basin of
a mode.
Attraction Basin
Slides by Y. Ukrainitz & B. Sarel

Mean Shift
region of interest
mean-shift vector
center of mass

• Kernel density estimation function
• Gaussian kernel
Kernel Density Estimation
ˆfh(x) =
1
nh
nX
i=1
K
✓
x xi
h
◆
K
✓
x xi
h
◆
=
1
p
2⇡
e
(x xi)2
2h2

• Compute mean shift vector m(x)
• Iteratively translate the kernel window y m(x)
until convergence
Computing the Mean Shift
m(x) =
2
4
Pn
i=1 xig
⇣
||x xi||2
h
⌘
Pn
i=1 g
⇣
||x xi||2
h
⌘ x
3
5

• Mean-shift can also be used as clustering-based
image segmentation.
Mean-Shift Segmentation
D. Comaniciu and P. Meer, Mean Shift: A Robust
Approach toward Feature Space Analysis, PAMI 2002.

• Compute features for each pixel (color, gradients,
texture, etc.).
• Set kernel size for features and position .
• Initialize windows at individual pixel locations.
• Run mean shift for each window until convergence.
• Merge windows that are within width of and .
Mean-Shift Segmentation
Color
space
Color
space
clusters
Kf Ks
Kf Ks

• Speedups:
• binned estimation
• fast neighbor search
• update each window in each iteration
• Other tricks
• Use kNN to determine window sizes
adaptively
Mean-Shift

• Pros
• Good general-practice segmentation
• Flexible in number and shape of regions
• robust to outliers
• Cons
• Have to choose kernel size in advance
• Not suitable for high-dimensional features
Mean-Shift pros & cons

• DBSCAN: Density-based spatial
clustering of applications with noise.
• Density: number of points within a
speciﬁed radius (ε-Neighborhood)
• Core point: a point with more than
a speciﬁed number of points
(MinPts) within ε.
• Border point: has fewer than
MinPts within ε, but is in the
neighborhood of a core point.
• Noise point: any point that is not a
core point or border point.
DBSCAN
MinPts=4
p is core point
q is border point
o is noise point
q p
"
"
o

• Density-reachable: p is density-
reachable from q w.r.t. ε and
MinPts if there is a chain of
objects p1, ..., pn with p1=q and
pn=p, s.t. pi+1 is directly density-
reachable from pi w.r.t. ε and
MinPts for all
• Density-connectivity: p is
density-connected to q w.r.t. ε
and MinPts if there is an object
o, s.t. both p and q are density-
reachable from o w.r.t. ε and
MinPts.
DBSCAN
1  i  n

• Cluster: a cluster C in a set of objects D w.r.t. ε
and MinPts is a non-empty subset of D satisfying
• Maximality: for all p,q, if p ∈ C and if q is
density reachable from p w.r.t. ε.
• Connectivity: for all p,q ∈ C, p is density-
connected to q w.r.t. ε and MinPts in D.
• Note: cluster contains core & border points.
• Noise: objects which are not directly density-
reachable from at least one core object.
DBSCAN clustering

1. Select a point p
2. Retrieve all points density-reachable from p
w.r.t. ε and MinPts.
1. if p is a core point, a cluster is formed
2. if p is a border point, no points are density
reachable from p and DBSCAN visits the
next point of the database
3. continue 1,2, until all points are processed.
(result independent of process ordering)
DBSCAN clustering algorithm

• Heuristic: for points in a cluster, their kth nearest
neighbors are at roughly the same distance.
• Noise points have the kth nearest neighbor at
farthest distance.
• So, plot sorted distance of every point to its kth
nearest neighbor.
DBSCAN parameters
sharp change;
good candidate
for ε and MinPts.

• Pros
• No need to decide K beforehand,
• Robust to noise, since it doesn't require every
point being assigned nor partition the data.
• Scales well to large datasets with .
• Stable across runs and different data ordering.
• Cons
• Trouble when clusters have different densities.
• ε may be hard to choose.
DBSCAN pros & cons

• Agglomerative clustering v.s. Divisive clustering
Hierarchical Clustering

• Method:
1. Every point is its own cluster
2. Find closest pair of clusters, merge into one
3. repeat
• The deﬁnition of closest is what differentiates
various ﬂavors of agglomerative clustering
algorithms.
Agglomerative Clustering

• How to deﬁne the linkage/cluster similarity?
• Maximum or complete-linkage clustering
(a.k.a., farthest neighbor clustering)
• Minimum or single linkage clustering (UPGMA)
(a.k.a., nearest neighbor clustering)
• Centroid linkage clustering (UPGMC)
• Minimum Energy Clustering
• Sum of all intra-cluster variance
• Increase in variance for clusters being merged
single linkage complete linkage average linkage centroid linkage

• How many clusters?
• Clustering creates a dendrogram (a tree)
• Threshold based on max number of clusters or
based on distance between merges.

• Pros
• Simple to implement, widespread application
• Clusters have adaptive shapes
• Provides a hierarchy of clusters
• Cons
• May have imbalanced clusters
• Still have to choose the number of clusters or
thresholds
• Need to use an ultrametric to get a meaningful
hierarchy

• Group points based on links in a graph
Spectral Clustering
A
B

• Normalized Cut
• A cut in a graph that penalizes large
segments
• Fix by normalizing for size of segments 
 
 
 
 
volume(A) = sum of costs of all edges that
touch A
Spectral Clustering
Normalized Cut(A, B) =
cut(A, B)
volume(A)
+
cut(A, B)
volume(B)

• Determining importance by random walk
• What's the probability of visiting a given node?
• Create adjacency matrix based on visual similarity
• Edge weights determine probability of transition
Visual Page Rank
Jing Baluja 2008

• Quantization/Summarization: K-means
• aims to preserve variance of original data
• can easily assign new point to a cluster
Which Clustering Algorithm to use?
Quantization for computing
histograms
Summary of 20,000 photos of Rome using “greedy k-means”
http://grail.cs.washington.edu/projects/canonview/

• Image segmentation: agglomerative clustering
• More ﬂexible with distance measures (e.g.,
can be based on boundry prediction)
• adapts better to speciﬁc data
• hierarchy can be useful
http://www.cs.berkeley.edu/~arbelaez/UCM.html

• K-means useful for
summarization, building
dictionaries of patches,
general clustering.
• Agglomerative clustering
useful for segmentation,
general clustering.
• Spectral clustering useful for
determining relevance,
summarization, segmentation.

• Synthetic dataset
Clustering algo. compared
http://hdbscan.readthedocs.io/en/latest/comparing_clustering_algorithms.html

• K-means, k=6

• Meanshift

• DBSCAN, ε=0.025

• Agglomerative Clustering, k=6, linkage=ward

• Spectral Clustering, k=6

Classiﬁcation
Dimension
Reduction
Clustering
continuous
discrete

• Given a set of samples and their ground
truth annotation , learn a function
that minimizes the prediction error
for new .
• The function is a classiﬁer. Classiﬁers
divides input space into decision regions
separated by decision boundaries.
Supervised Learning
xj /2 X
xi 2 X
yi
decision boundary
E(yj, f(xj))
y = f(x)
y = f(x)
x1
x2
R1
R2
R3

• Spam detection:
• X = { characters and words in the email }
• Y = { spam, not spam}
• Digit recognition:
• X = cut out, normalized images of digits
• Y = {0,1,2,3,4,5,6,7,8,9}
• Medical diagnosis
• X = set of all symptoms
• Y = set of all diseases
Supervised Learning Examples

• Find a linear function to separate the classes
Linear Classiﬁers
• Logistic Regression
• Naïve Bayes
• Linear SVM

• Using a probabilistic approach to model data,
the distribution of P(X,Y): given data X, ﬁnd the Y
that maximizes the posterior probability p(Y|X).
• Problem: we need to model all p(X|Y) and p(Y).
If | X | = n, there are 2n possible values for X.
• The Naïve Bayes' assumption assumes that xi's
are conditionally independent.
Naïve Bayes Classiﬁer
p(Y |X) =
p(X|Y )p(Y )
p(X)
, where p(X) =
X
Y
p(X|Y )p(Y )
p(X1 . . . Xn|Y ) =
Y
i
p(Xi|Y )

• Given:
• Prior p(Y)
• n conditionally independent features,
represented by the vector X, given the class Y
• For each Xi, we have likelihood p(Xi | Y)
• Decision rule:
Y ⇤
= argmax
Y
p(Y )p(X1, . . . , Xn|Y )
= argmax
Y
p(Y )
Y
i
p(Xi|Y )

• For discrete Naïve Bayes, simply count:
• Prior:
• Likelihood:
• Naïve Bayes Model:
Maximum Likelihood for Naïve Bayes
p(Y = y0
) =
Count(Y = y0
)
P
y Count(Y = y)
p(Xi = x0
|Y = y0
) =
Count(Xi = x0
, Y = y0
)
P
x Count(Xi = x, Y = y)
p(Y |X) / p(Y )
Y
i,j
p(X|Y )

• Conditional probability model over:
• Classiﬁer:
p(Ck|x1, . . . , xn) =
1
Z
p(Ck)
nY
i=1
p(xi|Ck)
˜y = argmax
k2{1,...,K}
p(Ck)
nY
i=1
p(xi|Ck)

• Features X are entire document. Xi for ith word in
article. X is huge! NB assumption helps a lot!
Naïve Bayes for Text Classiﬁcation

• Typical additional assumption: Xi's position in
document doesn't matter: bag of words.
aardvark 0
about 2
all 2
Africa 1
apple 0
...
gas 1
...
oil 1
...
Zaire 0

• Learning Phase:
• Prior: p(Y), count how many documents in
each topic (prior).
• Likelihood: p(Xi|Y), for each topic, count how
many times a word appears in documents of
this topic.
• Testing Phase: for each document, use Naïve
Bayes' decision rule:
argmax
y
p(y)
wordsY
i=1
p(xi|y)

• Given 1000 training documents from each
group, learn to classify new documents
according to which newsgroup it came from.
• comp.graphics,
• comp.os.ms-windows.misc
• ...
• soc.religion.christian
• talk.religion.misc
• ...
• misc.forsale
• ...

• Usually, features are not conditionally independent:
• Actual probabilities p(Y|X) often bias towards 0 or 1
• Nonetheless, Naïve Bayes is the single most used
classiﬁer.
• Naïve Bayes performs well, even when
assumptions are violated.
• Know its assumptions and when to use it.
Naïve Bayes Classiﬁer Issues
p(X1, . . . , Xn|Y ) 6=
Y
i
p(Xi|Y )

• Regression model for which the dependent
variable is categorical.
• Binomial/Binary Logistic Regression
• Multinomial Logistic Regression
• Ordinal Logistic Regression (categorical, but
ordered)
• Substituting Logistic Function 
 
, 
we get:
Logistic Regression
y(x, w) =
1
1 + e (w0+w1x)
˜x = w0 + w1xf(˜x) =
1
1 + e ˜x

• E.g., for predicting:
• mortality of injured patients,
• risk of developing a certain disease based on
observations of the patient,
• whether an American voter would vote
Democratic or Republican,
• probability of failure of a given process, system or
product,
• customer's propensity to purchase a product or
halt a subscription,
• likelihood of homeowner defaulting on mortgage.
When to use logistic regression?

• Hours studied vs passing the exam
Logistic Regression Example
Ppass(h) =
1
1 + e ( 4.0777+1.5046·h)

• Learn p(Y|X) directly. Reuse
ideas from regression, but let y-
intercept deﬁne the probability.
• With normalization
Logistic Regression Classiﬁer
p(Y = 1|X, w) / exp(w0 +
X
i
wiXi)
Exponential function
Logistic function
p(Y = 0|X, w) =
1
1 + exp(w0 +
P
i wiXi)
p(Y = 1|X, w) =
exp(w0 +
P
i wiXi)
1 + exp(w0 +
P
i wiXi)
y =
1
1 + exp( x)

• Prediction: output the Y with highest p(Y|X). For
binary Y, output Y if
Logistic Regression: decision boundary
p(Y = 0|X, w) =
1
1 + exp(w0 +
P
i wiXi)
p(Y = 1|X, w) =
exp(w0 +
P
i wiXi)
1 + exp(w0 +
P
i wiXi)
1 <
P(Y = 1|X)
P(Y = 0|X)
1 < exp(w0 +
nX
i=1
wiXi)
0 < w0 +
nX
i=1
wiXi
w0 + w · X = 0

• Decision boundary: p(Y=0 | X, w) = 0.5
• Slope of the line deﬁnes how quickly probabilities go to 0
or 1 around decision boundary.
Visualizing p(Y = 0|X, w) =
1
1 + exp(w0 + w1x1)

• Decision boundary is deﬁned by y=0 hyperplane
Visualizing p(Y = 0|X, w) =
1
1 + exp(w0 + w1x1 + w2x2)

• Generative (Naïve Bayes) loss function:
• Data likelihood
• Discriminative (logistic regression) loss function:
• Conditional Data likelihood
• Maximize conditional log likelihood!
Logistic Regression Param. Estimation
ln p(D|w) =
NX
j=1
ln p(xj
, yj
|w) =
NX
j=1
ln p(yj
|xj
, w) +
NX
j=1
ln p(xj
|w)
ln p(DY |DX, w) =
NX
j=1
ln p(yj
|xj
, w)

• Maximize conditional log likelihood (Maximum
Likelihood Estimation, MLE):
• No closed-form solution.
• Concave function of w → no need to worry
about local optima; easy to optimize.
l(w) ⌘ ln
Y
j
p(yj
|xj
, w)
=
X
j
yj
(w0 +
X
i
wixj
i ) ln(1 + exp(w0 +
X
i
wixj
i )

• Conditional likelihood for logistic regression is convex!
• Gradient:
• Gradient Ascent update rule:
• Simple, powerful, use in 
many places.
rwl(w) =

dl(w)
dw0
, . . . ,
dl(w)
dwn
w = ⌘rwl(w)
w
(t+1)
i w
(t)
i + ⌘
dl(w)
dwi

• MLE tends to prefer large weights
• Higher likelihood of properly classified
examples close to decision boundary.
• Larger influence of corresponding features on
decision.
• Can cause overfitting!!!

• Regularization to avoid large weights, overﬁtting.
• Add priors on w and formulate as Maximum a
Posteriori (MAP) optimization problem.
• Deﬁne prior with normal distribution, zero
mean, identity towards zero; pushes
parameters towards zero.
• MAP estimate:
p(w|Y, X) / p(Y |X, w)p(w)
w⇤
= argmax
w
ln
2
4p(w)
NY
j=1
p(yj
|xj
, w)
3
5

• Logistic Regression in more general case, where
Y = { y1, ..., yR}. Deﬁne a weight vector wi for
each yi, i=1,...,R-1.
Logistic Regression for Discrete Classiﬁcation
p(Y = 1|X) / exp(w10 +
X
i
w1iXi)
p(Y = 2|X) / exp(w20 +
X
i
w2iXi)
p(Y = r|X) = 1
r 1X
j=1
p(Y = j|X)
...

• E.g., Y={0,1}, X = <X1, ..., Xn>, Xi continuous.
Naïve Bayes vs Logistic Regression
Naïve Bayes
(generative)
Logistic Regression
(discriminative)
Number of parameters 4n+1 n+1
parameter estimation uncoupled coupled
when # training samples → infinite 
& model correct
good classifier good classifier
when # training samples → infinite 
& model incorrect
biased classifier
less-biased
classifier
Training samples needed O(log N) O (N)
Training convergence speed faster slower

Naïve Bayes vs Logistic Regression
• Examples from UCI Machine Learning dataset

Perceptron
• Invented in 1957 at the Cornell Aeronautical
Lab. Intended to be a machine instead of a
program that is capable of recognition.
• A linear (binary) classiﬁer.
Mark I
perceptron machine
i1
i2
in
...
+ f o
o = f
nX
k=1
ik · wk
!

• Start with zero weights: w=0
• For t=1...T (T passes over data)
• For i=1...n (each training sample)
• Classify with current weights 
(sign(x) is +1 if x>0, else -1)
• If correct, (i.e., y=yi), no change!
• If wrong, update
Binary Perceptron Algorithm
w = w + yi
xi
y = sign(w · xi
)
w xi
w + (-1) xi

Binary Perceptron example
update = 0

update = 1update = 1

update = 1update = 1update = 2

update = 1update = 1update = 2update = 3

update = 1update = 1update = 2update = 3update = 5

update = 1update = 1update = 2update = 3update = 5update = 10

update = 1update = 1update = 2update = 3update = 5update = 10update = 20

• If we have more than two classes:
• Have a weight vector for each class wy
• Calculate an activation function for each class
• Highest activation wins
Multiclass Perceptron
activationw(x, y) = wy · x
y⇤
= argmax
y
(activationw(x, y))

• Starts with zero weights
• For t=1, ..., T, i=1, ..., n (T times over data)
• Classify with current weights
• If correct (y=yi), no change!
• If wrong: subtract features xi from weights for
predicted class wy and add them to weights
for correct class wyi.
Multiclass Perceptron
y = argmax
y
wy · xi
wy = wy xi
wyi = wyi xi
xi
wyi
wyi + xi
wy
wy xi

• Text classiﬁcation example:
x = "win the vote" sentence
Multiclass Perceptron Example
BIAS 1
win 1
game 0
vote 1
the 1
,,,
BIAS -2
win 4
game 4
vote 0
the 0
,,,
BIAS 1
win 2
game 0
vote 4
the 0
,,,
BIAS 2
win 0
game 2
vote 0
the 0
,,,
wsports
wpolitics
wtech
x
x · wsports = 2
x · wpolitics = 7
x · wtech = 2
Classiﬁed as "politics"

• The data is linearly separable with margin if
Linearly separable (binary)
9w 8t yt
(w · xt
) > 0
x1
x2

• Assume data is separable with margin
• Also assume there is a number R such that
• Theorem: the number of mistakes (parameter
updates) made by the perceptron is bounded:
Mistake Bound for Perceptron
9w⇤
s.t.||w⇤
||2 = 1 and 8t yt
(w⇤
·t
)
8t ||xt
||2  R
mistakes 
R2
r2

• Noise: if the data isn't separable,
weights might thrash (averaging
weight vectors over time can help).
• Mediocre generalization: ﬁnds a
barely separating solution.
• Overtraining: test / hold-out
accuracy usually rises then falls.
Issues with Perceptrons
Seperable: Non-Seperable:
thrashing
barely separable

Linear SVM Classiﬁer
f(x) = g(w · x + b)
• Deﬁne hyperplane where is the
tangent to hyperplane, is the matrix of all
data points. Minimize s.t.
produces correct label for all .
t
X
tX b = 0
||t|| tX b
X
x1
x2

Linear SVM Classiﬁer
x1
x2 f(x) = g(w · x + b)
• Deﬁne hyperplane where is the
tangent to hyperplane, is the matrix of all
data points. Minimize s.t.
produces correct label for all .
t
X
tX b = 0
||t|| tX b
X
support vectors

• Some data sets are not linearly separable!
• Option 1:
• Use non-linear features, e.g., polynomial basis
functions
• Learn linear classifers in a transformed, non-
linear feature space
• Option 2:
• Use non-linear classiﬁers (decision trees,
neural networks, nearest neighbors)
Nonlinear Classiﬁers

• Assign label of nearest training data point to
each test data point.
Nearest Neighbor Classiﬁer
Duda, Hart and Stork, Pattern Classiﬁcation

K-Nearest Neighbor Classiﬁer
x x
x
x
x
x
x
x
o
o
o
o
o
o
o
x2
x1
+
+
x x
x
x
x
x
x
x
o
o
o
o
o
o
o
x2
x1
+
+
1-nearest
x x
x
x
x
x
x
x
o
o
o
o
o
o
o
x2
x1
+
+
3-nearest
x x
x
x
x
x
x
x
o
o
o
o
o
o
o
x2
x1
+
+
5-nearest

• Data that are linearly separable work out great:
• But what if the dataset is just too hard?
• We can map it to a higher-dimensional space!
Nonlinear SVMs
0
0
x
x
0
x
x2

• Map the input space to some higher dimensional
feature space where the training set is
separable:
Nonlinear SVMs
: x ! (x)

• The kernel trick: instead of explicitly computing
the lifting transformation
• This gives a non-linear decision boundary in the
original feature space:
• Common kernel function: Radial basis function
kernel.
Nonlinear SVMs
K(xi, xj) = (xi) · (xj)
X
i
↵iyi (xi) · (x) + b =
X
i
↵iyiK(xi, x) + b

• Consider the mapping:
Nonlinear kernel example
0
x
x2
(x) = (x, x2
)
(x) · (y) = (x, x2
) · (y, y2
) = xy + x2
y2
K(x, y) = xy + x2
y2

• Histogram intersection kernel:
• Generlized Gaussian kernel:
D can be (inverse) L1 distance, Euclidean
distance, distance, etc.
Kernels for bags of features
I(h1, h2) =
NX
i=1
min(h1(i), h2(i))
K(h1, h2) = exp
✓
1
A
D(h1, h2)2
◆
X2

• Combine multiple two-class SVMs
• One vs others:
• Training: learn an SVM for each class vs the others.
• Testing: apply each SVM to test example and
assign it to the class of the SVM that returns the
highest decision value.
• One vs one:
• Training: learn an SVM for each pair of classes
• Testing: each learned SVM votes for a class to
assign to the test example.
Multi-class SVM

• Pros:
• SVMs work very well in practice, even with very
small training sample sizes.
• Cons:
• No direct multi-class SVM; must combine two-class
SVMs.
• Computation and memory usage:
• Must compute matrix of kernel values for each
pair of examples.
• Learning can take a long time for large problems.
SVMs: Pros & Cons

• Prediction is done by sending the example down
the tree until a class assignment is reached.
Decision Tree Classiﬁer

• Internal Nodes: each test a feature
• Leaf nodes: each assign a classiﬁcation
• Decision Trees divide the feature space into axis-
parallel rectangles and label each rectangle with
one of the K classes.
Decision Tree Classiﬁer

• Goal: ﬁnd a decision tree that achieves minimum
misclassiﬁcation errors on the training data.
• Brute-force solution: create a tree with one path
from root to leaf for each training sample. 
(problem: just memorizing, won't generalize.)
• Find the smallest tree that minimizes error. 
(problem: this is NP-hard.)
Training Decision Trees

1. Choose the best feature a* for the root of the tree.
2. Split training set S into subsets {S1, S2, ..., Sk}
where each subset Si contains examples having
the same value for a*.
3. Recursively apply the algorithm on each new
subset until all examples have the same class
label.
The problem is, what deﬁnes the "best" feature?
Top-down induction of Decision Tree

• Decision Tree feature selection based on
classiﬁcation error.
Choosing Best Feature
Does not work well, since it doesn't reﬂect progress
towards a good tree.

• Choose feature that gives the highest
information gain (X that has the highest mutual
information with Y).
• Deﬁne to be the expected remaining
uncertainty about y after testing xj.
Choosing Best Feature
argmax
j
I(Xj; Y ) = argmax
j
H(Y ) H(Y |Xj)
= argmin
j
H(Y |Xj)
˜J(j)
˜J(j) = H(YX)j) =
X
x
p(Xj = x)H(Y |Xj = x)

Ensembles: Combining Classiﬁers

1. Create T bootstrap samples, {S1, ..., ST} of S as
follows:
• For each Si, randomly draw |S| examples from
S with replacement.
• With large |S|, each Si will contain 1 - 1/e =
63.2% unique examples.
2. For each i=1, ..., T, hi = Learn (Si)
3. Output H = <{h1, ..., hT}, majority vote >
Bootstrap Aggregating (Bagging)
Leo Breiman, "Bagging Predictors", Machine Learning, 24, 123-140 (1996)

• A learning algorithm is unstable if small changes
in the training data produces large changes in
the output hypothesis.
• Bagging will have little beneﬁt when used with
stable learning algorithms.
• Bagging works best when used with unstable
yet relatively accurate classiﬁers.
Learning Algorithm Stability

• Bagging: individual classifiers are independent
• Boosting: classifiers are learned iteratively
• Look at errors from previous classifiers to
decide what to focus on for the next iteration
over data.
• Successive classifiers depends upon its
predecessors.
• Result: more weights on "hard" examples, i.e.,
the ones classified incorrectly in the previous
iterations.
Boosting

• Consider E = <{h1, h2, h3}, majority vote>
• If h1, h2, h3 have error rates less than e, the error
rate of E is upper-bounded by g(a): 3e2-2e3 < e
Error Upper Bound
e
3e2-2e3

• Hypothesis of getting a classifier ensemble of
arbitrary accuracy, from weak classifiers.
Arbitrary Accuracy from Weak Classifiers
The original formulating of boosting learns too slowly.
Empirical studies show that Adaboost is highly effective.

• Adaboost works by learning many times on
different distributions over the training data.
• Modify learner to take distribution as input.
1. For each boosting round, learn on data set S
with distribution Dj to produce jth ensemble
member hj.
2. Compute the j+1th round distribution Dj+1 by
putting more weight on instances that hj made
mistake on.
3. Compute a voting weight wj for hj.
Adaboost

Adaboost Example
Credit: "A tutorial on boosting" by Yoav Freund and Rob Schapire.

• Suppose the base learner L is a weak learner,
with error rate slightly less than 0.5 (better than
random guess)
• Training error goes to zero exponentially fast!!!
Adaboost Properties

Semi-supervised Learning
Dimension
Reduction
Clustering
continuous
discrete

• Assume that class boundary should go through
low density areas.
• Having unlabeled data helps getting better
decision boundary.
Why can unlabeled data help?
supervised learning
semi-supervised learning

• Assume that each
class contains a
coherent group of
points (e.g., Gaussian)
• Having unlabeled data
points can help learn
the distribution more
accurately.
Why can unlabeled data help?

• Generative models:
• Use unlabeled data to more accurately
estimate the models.
• Discriminative models:
• Assume that p(y|x) is locally smooth
• Graph/manifold regularization
• Multi-view approach: multiple independent
learners that agree on unlabeled data
• Cotraining
Semi-Supervised Learning (SSL)

SSL Bayes Gaussian Classiﬁer
Without SSL:
optimize
With SSL:
optimize
p(Xl, Yl|✓)
p(Xl, Yl, Xu|✓)

• In SSL, the learned needs to explain the
unlabeled data well, too.
• Find MLE or MAP estimate of joint and marginal
likelihood:
• Common mixture models used in SSL:
• GMM
• Mixture of Multinomials
SSL Bayes Gaussian Classiﬁer
✓
p(Xl, Yl, Xu|✓) =
X
Yu
p(Xl, Yl, Xu, Yu|✓)

• Binary classiﬁcation with GMM using MLE
• Using labeled data only, MLE is trivial:
• With both labeled and unlabeled data, MLE is
harder---use EM:
Estimating SSL GMM params
log p(Xl, Yl|✓) =
lX
i=1
log p(yi|✓) p(xi|yi, ✓)
+
l+uX
i=l+1
log (
2X
y=1
p(y|✓) p(xi|y, ✓))
log p(Xl, Yl|✓) =
lX
i=1
log p(yi|✓) p(xi|yi, ✓)

• Start with MLE
• = proportion of class c
• = sample mean of class c
• = sample covariance of class c
• The E-step: compute the expected label 
 
 
for all .
• The M-step: update MLE with (now labeled)
Semi-Supervised EM for GMM
✓ = {w, µ, ⌃}1:2 on (Xl, Yl)
wc
µc
⌃c
p(y|x, ✓) =
p(x, y|✓)
P
y0 p(x, y0|✓)
x 2 Xµ
✓ Xµ

• SSL is sensitive to assumptions!!!
• Cases when the assumption is wrong:
SSL GMM Discussions

-- 蘇軾 <<稼說>>
博觀而約取
厚積而薄發

Machine Learning Foundations at Taiwan AI Academy

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