1. SAMPLING Obtaining a Sample From a Population

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

4. The population for a study about UNI students is all the students at UNI.

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

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

7. A CENSUS is a sample of an entire Population…. They are seldom done because….. 1. Impossible

9. A CENSUS is a sample of an entire Population…. They are seldom done because….. 1. Impossible 2. Unprofitable

10. A CENSUS is a sample of an entire Population…. They are seldom done because….. 1. Impossible 2. Unprofitable 3. Less accurate

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

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

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

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

16. Sampling is a five step process:

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

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

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

20. 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. Male Female Male Female France India

21. 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.

22. 2. Identify the sampling frame

23. A sampling frame is a list of the units of a population: telephone numbers addresses email addresses

25. 3. Select a sampling procedure

26. Two kinds: 1. Nonprobability samples 2. Probability samples

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

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

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

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

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

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

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

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

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

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

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

41. 4. Determine a sample size

42. What is the correct sample size? The answer is always the same. IT DEPENDS!

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

44. 4. Determine a sample size And what does it depend upon? 1. Sampling error http://www.rasmussenreports.com/public_content/politics/obama_admi nistration/daily_presidential_tracking_poll

45. 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

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

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

48. 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

49. 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

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

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

52. H = Z SE

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

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

55. H = ZSE

56. 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….

57. 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

58. 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:

59. What size sample would be needed to: 1.Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points?

60. What size sample would be needed to: 1.Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points? 2.A Likert scale to a 95% accuracy of 0.4?

61. What size sample would be needed to: 1.Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points? 2.A Likert scale to a 95% accuracy of 0.4? 3.Income scale with a SD = $15,000 to within a 95% accuracy of $5,000?

62. What size sample would be needed to: 1.Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points? 2.A Likert scale to a 95% accuracy of 0.4? 3.Income scale with a SD = $15,000 to within a 95% accuracy of $5,000? 4.The largest sample that would ever be needed to do an over-night political poll with a 95% accuracy of 2%?

63. What size sample would be needed to: 1.Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points? 2.A Likert scale to a 95% accuracy of 0.4? 3.Income scale with a SD = $15,000 to within a 95% accuracy of $5,000? 4.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%?

64. 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?

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

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

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

68. 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.

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

70. An example of the problems in sampling and the value of demographics…. http://www.dailymail.co.uk/news/article-1315078/Race-maps-America.html http://www.prb.org/Publications/Articles/2008/workingaroundtheclock.aspx