Teknik sampling

1. SAMPLING TECHNIQUE
AND SAMPLE SIZE
Kuliah minggu ke-12

2. Important statistical terms
Population:
a set which includes all
measurements of interest
to the researcher
(The collection of all
responses, measurements, or
counts that are of interest)
Sample:
A subset of the population

3. why sampling?
Get information about large populations
 Biaya sedikit
 Waktu sampling pendek
 More accuracy i.e. Can Do A Better Job of
Data Collection
 When it’s impossible to study the whole
population

4. Populasi Target :
The population to be studied/ to which the investigator
wants to generalize his results
Unit Sampling:
unit terkecil dari sampel yang dapat diambil
Sampling frame
List of all the sampling units from which sample is drawn
Sampling scheme
Method of selecting sampling units from sampling frame

5. Types of sampling
 Non-probability samples
 Probability samples

6. Non probability samples
 Convenience samples (ease of access)
sample is selected from elements of a population
that are easily accessible
 Snowball sampling (friend of friend….etc.)
 Purposive sampling (judgemental)
 You chose who you think should be in the
study
 Quota sample

7. Non probability samples
Probability of being chosen is unknown
Cheaper- but unable to generalise
potential for bias

8. Probability samples
 Random sampling
 Each subject has a known probability of being
selected
 Allows application of statistical sampling
theory to results to:
 Generalise
 Test hypotheses

9. Conclusions
 Probability samples are the best
 Ensure
 Representativeness
 Precision

10. Methods used in probability samples
 Simple random sampling
 Systematic sampling
 Stratified sampling
 Multi-stage sampling
 Cluster sampling

11. Simple random sampling

12. Table of random numbers
6 8 4 2 5 7 9 5 4 1 2 5 6 3 2 1 4 0
5 8 2 0 3 2 1 5 4 7 8 5 9 6 2 0 2 4
3 6 2 3 3 3 2 5 4 7 8 9 1 2 0 3 2 5
9 8 5 2 6 3 0 1 7 4 2 4 5 0 3 6 8 6

13. Sampling fraction
Ratio between sample size and population
size
Systematic sampling

14. Systematic sampling

15. Cluster sampling
Cluster: a group of sampling units close to each
other i.e. crowding together in the same area or
neighborhood

16. Cluster sampling
Section 4
Section 5
Section 3
Section 2
Section 1

17.  Stratified sampling
 Multi-stage sampling

18. Systematic error (or bias)
Inaccurate response (information bias)
Selection bias
Sampling error (random error)
Errors in sample

19. Type 1 error
 The probability of finding a difference with
our sample compared to population, and
there really isn’t one….
 Known as the α (or “type 1 error”)
 Usually set at 5% (or 0.05)

20. Type 2 error
 The probability of not finding a difference
that actually exists between our sample
compared to the population…
 Known as the β (or “type 2 error”)
 Power is (1- β) and is usually 80%

21. Sample size
Quantitative Qualitative
2
D
2
σ
2
Z
n 
2
2
2
2
1
D
)xF
σ
(σ
n


2
2
D
π)
π(1
Z
n


2
D
F
)
P
-
(1
P
2
n 

22. Problem 1
A study is to be performed to determine a
certain parameter in a community. From a
previous study a sd of 46 was obtained.
If a sample error of up to 4 is to be accepted.
How many subjects should be included in
this study at 99% level of confidence?

23. Answer
881
~
3
.
880
2
4
2
46
x
2
2.58
n 

2
D
2
σ
2
Z
n 

24. Problem 2
 A study is to be done to determine effect of 2
drugs (A and B) on blood glucose level. From
previous studies using those drugs, Sd of
BGL of 8 and 12 g/dl were obtained
respectively.
 A significant level of 95% and a power of
90% is required to detect a mean difference
between the two groups of 3 g/dl. How many
subjects should be include in each group?

25. Answer
group
each
in
243
~
6
.
242
3
)x10.5
12
(8
n 2
2
2



2
2
2
2
1
D
)xF
σ
(σ
n



26. Problem 3
It was desired to estimate proportion of
anaemic children in a certain preparatory
school. In a similar study at another school a
proportion of 30 % was detected.
Compute the minimal sample size required at a
confidence limit of 95% and accepting a
difference of up to 4% of the true population.

27. Answer
505
~
21
.
504
(0.04)
0.3)
0.3(1
x
1.96
n 2
2



2
2
D
π)
π(1
Z
n



28. Problem 4
In previous studies, percentage of
hypertensives among Diabetics was 70%
and among non diabetics was 40% in a
certain community.
A researcher wants to perform a comparative
study for hypertension among diabetics and
non-diabetics at a confidence limit 95% and
power 80%, What is the minimal sample to
be taken from each group with 4% accepted
difference of true value?

29. Answer
2
.
2413
0.04
x7.8
0.55)
-
(1
0.55
x
2
n 2


2
D
F
)
P
-
(1
P
2
n 

30. Precision
Cost