1. Obtaining a Sample From a Population
SAMPLING
Obtaining a Sample From a Population

2. A population is all the people or objects of
interest in a study.

3. Population data can be very informative, especially in business.
What can you determine about Iowa by
looking at the following overlay of
demographics on a map?

5. What do we learn about China by simply looking
at population data?

6. Statistically, the population for a study about UNI students
would be all the students at UNI.

7. If a study utilized everyone in a population,
the sample would be called a
Census.

8. A CENSUS is a sample of an
entire Population….
They are seldom done because…..

9. Impossible A CENSUS is a sample of an entire Population….
They are seldom done because…..
Impossible

11. Impossible Unprofitable A CENSUS is a sample of an entire Population….
They are seldom done because…..
Impossible
Unprofitable

12. Impossible Unprofitable Less accurate A CENSUS is a sample of an
entire Population….
They are seldom done because…..
Impossible
Unprofitable
Less accurate

13. So… instead of a census,
a sample is taken.

15. Sampling is the process of discovering what a population
is like….. but
You can never look directly at the population!

16. The population is like a large container… you can pull things out
but you are NEVER allowed to look at all its contents.

17. Sampling must be done properly or
sampling itself can invalidate your research!

19. Sampling is a five step process:

20. Sampling is a five step process
Each step is important 5

21. Sampling is a five step process Each step is important
Or the sample will not give
an accurate estimate of the
population.

22. Before a sample can be taken from a
population: that population must
be defined.

23. 1. Before a sample can be taken from a population
that population must be defined.
A population: The total member of objects or
persons that are of interest for a study.
India
France
Male Female Male Female

24. 1. Before a sample can be taken from a population
that population must be defined.
A population: The total member of objects or
persons that are of interest for a study.
The characteristics of a population are called:
Parameters.

25. 2. Identify the sampling frame

26. 2. Identify the sampling frame
A sampling frame is a list of the units of a
population:
telephone numbers
addresses
addresses
commercial sources
“Big” data

28. 3. Select a sampling procedure

29. 3. Select a sampling procedure
Two kinds:
1. Nonprobability samples
2. Probability samples

30. 3. Select a sampling procedure
1. Nonprobability samples

31. 3. Select a sampling procedure
1. Nonprobability samples
Convenience Sample
(Accidental Sample)

32. 3. Select a sampling procedure
1. Nonprobability samples
Convenience Sample
(Accidental Sample)
Judgmental Sample

33. 3. Select a sampling procedure
1. Nonprobability samples
Convenience Sample
(Accidental Sample)
Judgmental Sample
Quota sample

35. 3. Select a sampling procedure
2. Probability samples

36. 3. Select a sampling procedure
2. Probability samples
Simple Random Sample
(SRS)

37. 3. Select a sampling procedure 2. Probability samples
Simple Random Sample
(SRS)
Special case of the convenience sample…..

38. 3. Select a sampling procedure
2. Probability samples
Simple Random Sample
(SRS)
Stratified Sample

40. 3. Select a sampling procedure
2. Probability samples
Simple Random Sample
(SRS)
Stratified Sample
Cluster Sample

41. Cluster Sample

42. 3. Select a sampling procedure
2. Probability samples
Simple Random Sample
(SRS)
Stratified Sample
Cluster Sample
Systematic Sample

43. The human mind will not randomize!
Note: Important!
The human mind will not randomize!

44. Select a sampling procedure

46. 4. Determine a sample size

47. IT DEPENDS! 4. Determine a sample size
What is the correct sample size?
The answer is always the same.
IT DEPENDS!

48. 4. Determine a sample size
And what does it depend upon?

49. 4. Determine a sample size And what does it depend upon?
1. Sampling error

50. 4. Determine a sample size And what does it depend upon?
1. Sampling error
National Survey of 1,000 Likely Voters Conducted September 20-21, 2010
By Rasmussen Reports
1* Is the current structure of the Social Security system a Ponzi scheme?
27% Yes
36% No
37% Not sure
NOTE: Margin of Sampling Error, +/- 3 percentage points with a 95% level of confidence

51. 4. Determine a sample size
And what does it depend upon?
1. Sampling error
2. Precision

52. 4. Determine a sample size
And what does it depend upon?
1. Sampling error
2. Precision
3. Degree of confidence

53. 4. Determine a sample size
And what does it depend upon?
1. Sampling error
2. Precision
3. Degree of confidence
4. Other factors
a. time
b. cost
c. type of scales
d. number of cross-classifications

54. 4. Determine a sample size
And what does it depend upon?
1. Sampling error
2. Precision
3. Degree of confidence
4. Other factors
f. historical and secondary data
g. statistical considerations
h. political considerations

55. 4. Determine a sample size
Since any given interval on the normal curve can
be calculated by:

56. 4. Determine a sample size
Since any given interval on the normal curve can
be calculated by:

57. H = Z SE

58. Since any given interval on the normal curve can
be calculated by:
The sample size can then be seen as:

59. The sample size for nominal and ordinal data
can then be seen as:

60. H = ZSE

61. What size sample would be needed if you wished to
make a decision to within 2 IQ point with 95% accuracy?
IQ has a standard deviation of 15…… therefore:
With Z = 2, H = 2; therefore….

62. It is assumed that 55% of all students are women, what size sample would be
needed to test this assumption to a 95% accuracy at 2%?
p = .55, q = .45 (p + q = 1); H = .02; Z = 2

63. EVER needed is: The largest sample size that will ever be needed
for any given H is when nominal data is being
used and p = q = 0.5. Therefore the largest sample
EVER needed is:

64. What size sample would be needed to:
Use an IQ scale with a SD = 15 to an accuracy of 95% at 3 IQ points?

65. What size sample would be needed to:
Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points?
A Likert scale to a 95% accuracy of 0.4?

66. What size sample would be needed to:
Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points?
A Likert scale to a 95% accuracy of 0.4?
Income scale with a SD = $15,000 to within a 95% accuracy of $5,000?

67. What size sample would be needed to:
Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points?
A Likert scale to a 95% accuracy of 0.4?
Income scale with a SD = $15,000 to within a 95% accuracy of $5,000?
The largest sample that would ever be needed to do an over-night political
poll with a 95% accuracy of 2%?

68. What size sample would be needed to:
Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points?
A Likert scale to a 95% accuracy of 0.4?
Income scale with a SD = $15,000 to within a 95% accuracy of $5,000?
The largest sample that would ever be needed to do an over-night political
poll with a 95% accuracy of 2%?
5. To test a hypothesis that 80% of all customers will buy a product again to
within 95% accuracy at 3%?

69. What is the sample size? Why is it +/- 3 percent at 95% confidence?
National Survey of 1,000 Likely Voters Conducted September 20-21, 2010
By Rasmussen Reports
1* Is the current structure of the Social Security system a Ponzi scheme?
27% Yes
36% No
37% Not sure
NOTE: Margin of Sampling Error, +/- 3 percentage points with a 95% level of confidence
Why is it +/- 3 percent at 95% confidence?
What is the sample size?

70. 4. Determine a sample size
What about the size of the population?

71. 4. Determine a sample size
What about the size of the population?
Does that change the size of the sample
needed?

72. 4. Determine a sample size
Surprisingly so,
Usually not!!

73. 4. Determine a sample size
Pragmatically, the sample size is independent
of the population size!
Except for very small populations, or very, very
large samples.

74. 5. Collect data (go do it!!)

75. An example of the problems in sampling and the value of demographics….