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Psychology 242, Dr. McKirnan

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Psychology 242, Dr. McKirnan

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    1. Psychology 242, Dr. McKirnan Week 3; Experimental designs Psychology 242, Dr. McKirnan Define your target population Operationally define your criteria for population “membership” Define your sampling frame Probability sampling Non-probability sampling

    2. Psychology 242, Dr. McKirnan Week 3; Experimental designs The big picture: Research sampling Define your target population Operationally define your criteria for population “membership” Define your sampling frame Probability sampling Non-probability sampling

    3. Psychology 242, Dr. McKirnan Week 6; Sampling Define the target population

    4. Psychology 242, Dr. McKirnan Week 6; Sampling Research samples & validity Clinical drug trials illustrate the conflict between internal v. external validity in sampling. People with diverse symptoms and backgrounds see physicians for depression. To enhance internal validity drug researchers use exclusion criteria to select only participants who fit a specific definition of depression Zimmerman et al. suggest that too many exclusion criteria compromises the validity of this research area. (click image for article)

    5. Psychology 242, Dr. McKirnan Week 6; Sampling Exclusion criteria & validity The study begins with a large # of people self-referred for depression

    6. Psychology 242, Dr. McKirnan Week 6; Sampling External validity in internally valid studies Applying rigorous study selection criteria for drug trials excludes the great majority of routine depression patients. Rigorous participant selection for internal validity seriously compromises external validity in these studies. This leaves the actual usefulness of anti-depressant (and other) medications for the general population in doubt. To be useful research must balance the need for careful subject selection with the need for representativeness

    7. Psychology 242, Dr. McKirnan Week 3; Experimental designs Research sampling Define your target population Operationally define your criteria for population “membership” Define your sampling frame Probability sampling Non-probability sampling

    8. Psychology 242, Dr. McKirnan Week 3; Experimental designs Question 1a

    9. Psychology 242, Dr. McKirnan Week 3; Experimental designs Question 1b

    10. Psychology 242, Dr. McKirnan Week 3; Experimental designs Question 1b

    11. Psychology 242, Dr. McKirnan Week 3; Experimental designs Question 1b

    12. Psychology 242, Dr. McKirnan Week 6; Sampling Major Steps in Sampling (2)

    13. Psychology 242, Dr. McKirnan Week 3; Experimental designs Research sampling Define your target population Operationally define your criteria for population “membership” Define your sampling frame Probability sampling Non-probability sampling

    14. Psychology 242, Dr. McKirnan Week 6; Sampling Major Steps in Sampling (3)

    15. Psychology 242, Dr. McKirnan Week 3; Experimental designs Research sampling Define your target population Operationally define your criteria for population “membership” Define your sampling frame Probability sampling Non-probability sampling

    16. Psychology 242, Dr. McKirnan Week 6; Sampling Major forms of sampling

    17. Psychology 242, Dr. McKirnan Week 6; Sampling Probability / Random Sampling Core feature: all members of the study population have = chance of inclusion in the sample Procedure: Choose participants from a table of random numbers, select every nth name from a list, call every nth # in a phone exchange, etc. Advantages: eliminates obvious biases of convenience sampling (below) Limitations: may undersample unusual / hard to reach participants Some may be unavailable in, e.g., telephone lists, computer files.

    18. Psychology 242, Dr. McKirnan Week 6; Sampling Basic Forms of random sampling Simple Random Sampling: Select a specific % of a target population; all members of population have about = chance of selection. Multi-Stage: Randomly select population units (census tracts, households, schools..), then randomly select individuals within unit. Stratified: Random within population sub-blocks, e.g., gender (randomly select 50 women and randomly select 50 men), ethnicity, etc. Cluster: Random within (potentially convenience) clusters, e.g., specific locations or “venues”, events, times of day, etc.

    19. Psychology 242, Dr. McKirnan Week 6; Sampling Random sampling Issues in Simple Random Sampling:

    20. Psychology 242, Dr. McKirnan Week 6; Sampling Multi-Stage Random Sampling

    21. Psychology 242, Dr. McKirnan Week 6; Sampling Stratified or cluster sampling Objective: Represent every key segment of the population. Procedure: decide which population segments are important (e.g. ethnic groups, census tracts, geographic areas...), randomly select from each segment. Proportionate: Same sampling fraction from each segment; approximates overall population (e.g., sample 1% of all African-Americans, 1% of all Latinos, etc…) Disproportionate: Unequal sampling fraction across segments, to over-represent smaller groups (e.g., select larger % of recent immigrants or other small groups…).

    22. Psychology 242, Dr. McKirnan Week 3; Experimental designs Research sampling Define your target population Operationally define your criteria for population “membership” Define your sampling frame Probability sampling Non-probability sampling

    23. Psychology 242, Dr. McKirnan Week 6; Sampling Non-Probability Sampling

    24. Psychology 242, Dr. McKirnan Week 6; Sampling Problem: No evidence for representativeness Advantage: availability of participants Non-Probability methods (1)

    25. Psychology 242, Dr. McKirnan Week 6; Sampling Non-Probability methods, 2 Sample a specific, well-defined group Assume group members are well represented at specific locations or settings (“venues”). “Intercept” methods for reaching participants Typically indigenous outreach workers Standard recruitment script Collect / distribute contact information for later participation Time / Space randomization: Lessen bias due to choice of venue by randomization: Venue x time Participants who are approached (e.g., every 4th…) Strategy must be based on a clear epidemiological or theory question.

    26. Psychology 242, Dr. McKirnan Week 3; Experimental designs

    27. Psychology 242, Dr. McKirnan Week 3; Experimental designs

    28. Psychology 242, Dr. McKirnan Week 6; Sampling Non-Probability methods, 3 Sample a specific, hard to reach group No census or similar data for sampling frame. Uses multiple (convenience) sampling “frames”: Direct outreach to places where population members are available (venue sampling). Newsletters, internet lists & chat rooms Organizations or meeting places Strategy must be based on a clear epidemiological or theory question. Most common & valid convenience sample

    29. Psychology 242, Dr. McKirnan Week 6; Sampling Non-Probability methods (3) Early participants are paid to recruit others, who recruit others, etc. Form of targeted sampling: Recruit network of “linked” people tracked by referrals Problem: Elicit correct proportions of group members? Advantage: Access unusual or “hidden” people related by a common behavior. With enough “generations” of links can well represent a target population. With RDS can show “chain” of referrals / links.

    30. Psychology 242, Dr. McKirnan Week 6; Sampling RDS coupon examples

    31. Psychology 242, Dr. McKirnan Week 6; Sampling RDS; chain description

    32. Psychology 242, Dr. McKirnan Week 6; Sampling Example of social network sampling: Bearman et al., Romantic ties among adolescents

    33. Psychology 242, Dr. McKirnan Week 6; Sampling Non-Probability methods (4)

    34. Psychology 242, Dr. McKirnan Week 6; Sampling Non-Probability methods, 5

    35. Psychology 242, Dr. McKirnan Week 6; Sampling Non-Probability methods (4) Sample every sector of a population -- at least several of everyone -- without worrying about proportions. Assume that a few people are a good proxy for the group.

    36. Psychology 242, Dr. McKirnan Week 6; Sampling Sampling overview

    37. Psychology 242, Dr. McKirnan Week 3; Experimental designs

    38. Psychology 242, Dr. McKirnan Week 6; Sampling Overview, 3

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