7 QC Tools

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7 QC Tools:
The Lean Six Sigma Pocket
Toolbook
•Flowchart [p. 33-41]
•Check Sheet [p. 78-81]
•Histogram [p. 111-113]
•Pareto [p. 142-144]
•Cause-and-Effect [p. 146-147]
•Scatter [p. 154-155]
•Control Chart [p. 122-135]
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Pareto Diagram
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Step 1: Decide on problem, type of
data, and causes or categories.
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Step 2: Collect the data.
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Process Variability Concepts 2

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Step 3: Order the causes or categories.
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Step 4: Calculate the cumulative totals.
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Process Variability Concepts 3

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Step 5: Draw and label the horizontal
axis.
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Step 6: Draw, scale, and
label the vertical axis.
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Process Variability Concepts 4

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Step 7: Draw bars for each cause or
category.
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Step 8: Draw cumulative total lines.
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Interpret the Pareto Chart.
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Pareto Diagram
(Using EXCEL)
1. Create a table listing the sources of defects in the first column
and in the second column calculate the total number of defects per
source.
Error Category Jan Feb Mar Apr May Jun Total
Improper credit check 2 1 1 4
Unsigned signature card 4 3 2 3 4 2 18
Starter checks not provided 4 1 1 6
Disclosures not provided 1 1 1 3
Checks not ordered 2 4 3 2 5 16
Paperwork lost at DP center 1 1 2
Incorrect data entry at DP 2 2 4
source: Brightman, Data Analysis
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2. Sort the table by the total number of defects in descending order.
In the third column, calculate the cumulative percentage for each row
in the table.
Error Category Total Error Category Total Cum %
Unsigned signature card 18 Unsigned signature card 18 33.96%
Checks not ordered 16 Checks not ordered 16 64.15%
Starter checks not provided 6 Starter checks not provided 6 75.47%
Improper credit check 4 Improper credit check 4 83.02%
Incorrect data entry at DP 4 Incorrect data entry at DP 4 90.57%
Disclosures not provided 3 Disclosures not provided 3 96.23%
Paperwork lost at DP center 2 Paperwork lost at DP center 2 100.00%
3. Create a chart with the ChartWizard (custom --- line-column
on two axes).
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Opening checking account errors
20 100.00%
15 80.00%
60.00%
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40.00%
5 20.00%
0 0.00%
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Process Variability Concepts 7

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Cause and Effect Diagram
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Step 1: Develop problem statement.
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Process Variability Concepts 8

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Step 2: Brainstorm causes.
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Step 2: Brainstorm causes.
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Step 3: Determine the major cause categories.
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Step 4: Determine the category for
Each listed cause.
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Step 4: Determine the category for
Each listed cause.
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Step 5: Put categories and causes
On cause & effect diagram.
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Step 6: Identify the most likely causes.
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“Failure to understand variation is
the central problem of
management.”
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Process Variability Concepts 12

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Stable vs. Unstable process
Stable process: a process in which variation in
outcomes arises only from common causes.
Unstable process: a process in which variation is a
result of both common and special causes.
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source: Moen, Nolan and Provost, Improving Quality Through Planned Experimentation
Red Bead experiment
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Red Bead Experiment
What are the lessons learned?
1.
2.
3.
4.
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Statistical Process Control:
Control Charts
Process
Parameter • Track process parameter over time
- mean
- percentage defects
Upper Control Limit (UCL)
• Distinguish between
- common cause variation
Center Line (within control limits)
- assignable cause variation
(outside control limits)
Lower Control Limit (LCL)
• Measure process performance:
how much common cause variation
is in the process while the process
Time is “in control”?
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Conceptual
view
of SPC
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source: Donald Wheeler, Understanding Statistical Process Control
Process
Stability
vs.
Process
Capability
Wheeler, Understanding Statistical Process Control 30
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Advantages of Statistical Control
1. Can predict its behavior.
2. Process has an identity.
3. Operates with less variability.
4. A process having special causes is unstable.
5. Tells workers when adjustments should not be made.
6. Provides direction for reducing variation.
7. Plotting of data allows identifying trends over time.
8. Identifies process conditions that can result in an
acceptable product.
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source: Juran and Gryna, Quality Planning and Analysis, p. 380-381.
Identifying Special Causes of Variation
source: Brian Joiner, Fourth Generation Management, pp. 260.
See also Lean Six Sigma
Pocket Toolbook, p. 133-135.
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Strategies for Reducing Special Causes of Variation
• Get timely data so special causes are signaled
quickly.
• Put in place an immediate remedy to contain any
damage.
• Search for the cause -- see what was different.
• Develop a longer term remedy.
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source: Brian Joiner, Fourth Generation Management, pp. 138-139.
“In a common cause
situation, there is no such
thing as THE cause.”
Brian Joiner
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Improving a Stable Process
• Stratify -- sort into groups or categories; look for
patterns. (e.g., type of job, day of week, time, weather,
region, employee, product, etc.)
• Experiment -- make planned changes and learn from
the effects. (e.g., need to be able to assess and learn
from the results -- use PDCA .)
• Disaggregate -- divide the process into component
pieces and manage the pieces. (e.g., making the
elements of a process visible through measurements
and data.)
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source: Brian Joiner, Fourth Generation Management, pp. 140-146.
A Conversation with Joseph Juran
“Take this example: In finance we set a budget. The actual expenditure, month by
month, varies - we bought enough stationery for three months, and that’s going to be
a miniblip in the figures. Now, the statistician goes a step further and says, ‘How do
you know whether it’s a miniblip or there’s a real change here?’ The statistician says,
‘I’ll draw you a pair of lines here. These lines are such that 95% of the time, you’re
going to get variation between them.’
Now suppose something happens that’s clearly outside the lines. The odds are
something’s amok. Ordinarily this is the result of something local, because the
system is such that it operates in control. So supervision converges on the scene to
restore the status quo.
Notice the distinction between what’s chronic [common cause] and what’s sporadic
[special cause]. Sporadic events we handle by the control mechanism. Ordinarily
sporadic problems are delegable because the origin and remedy are local. Changing
something chronic requires creativity, because the purpose is to get rid of the status
quo - to get rid of waste. Dealing with chronic requires structured change, which has
to originate pretty much at the top.”
Source: A Conversation with Joseph Juran, Thomas Stewart, Fortune, January 11, 1999, p. 168-170. 36
Process Variability Concepts 18