Subjects

1. Subjects

2. Sequential Questions • How many do I need? • Where do I find them? • How do I sample them? • How do I get them to take part in my study? • How do I get them to take the entire study?

3. How many do I need? • Power • Definition • ability to detect differences when they actually exist • chances of finding a significant effect • Ideally… • Conduct power analysis to determine sample size before you conduct your study • Want .80+ • In reality… • Rarely conduct unless grant proposal or IRB requests it. Sometimes conduct power post hoc. • Typically .40+

4. How many do I need? • Power • Many, many websites and programs to help you • G*Power http://www.psycho.uni-duesseldorf.de/abteilungen/aap/gpower3/ • Large list for every type of design http://www.statpages.org/#Power • See my handout

5. How many do I need? • Sample size • In reality… • Laboratory • 20+ minimum per cell • 40+ if all Ss in same cell • Online • 40+ minimum per cell • 100+ if all Ss in same cell • Mail/Telephone • ???

6. Where do I find them? • In-person • Undergraduate subject pools • Your classes; your colleagues classes • Field-study • In-person interviews • Online • Email • Online study • Telephone/mail • Professional survey company

8. How do I sample them? • Terminology • Population = the entire group of individuals you wish to describe • Sample = the group of individuals within your study • Problem #1 • If impractical/impossible to sample the entire Population, then how to sample to accurately describe (generalize) to Population. • Solution = Representative sampling via random sampling • Random sampling = Each member of the Population has an equal chance of being selected • How to randomly sample = Identify every member of the Population, assign them a number, and then use random number generator to pick the number you need Random number generator = http://www.random.org/

9. How do I sample them? • Problem #2 • If don’t have access to the entire Population, then can’t assign each one a number, and thus can’t ensure that each has equal chance of being selected. So how to sample limited set that is representative? • Solution = Proportionate stratified random sampling • Identify key variables of interest within Population • Identify percentages of key variables w/in Population • Randomly select the appropriate percentages from each subgroup • Thus, Sample matches Population on key variables

11. How do I sample them? • Problem #3 • If don’t have access to the entire SUBGROUP, then can’t assign each one a number, and thus can’t ensure that each has equal chance of being selected. So how to sample limited set that is representative? • Solution = Quota sampling • Non-random sampling where accept everyone into the study until reach quota or percentage • Thus, representative of key variables; but not randomly sampled so more variance (individual level error / extraneous confounds) on non-key variables.

12. How do I sample them? • Problem #4 • Don’t KNOW key variables or percentages of key variables within subgroup or Population • Don’t have ACCESS to subgroups or Population • Solution = Convenience sample -- the group of individuals chosen because of practical concerns like access and availability • Typically undergraduate psychology students who volunteer for extra credit, or online participants • Typically quota sampling where accept everyone into the study who is willing

13. How do I sample them? • Problem #5 • Convenience sample not representative, and not percentages based on key variables • Convenience sample suffers from self-selection bias (e.g., only some students want extra credit, only those with computers can take study online, etc) • Solution = ??? In general, field has accepted need for convenience sample (publish or perish) so only criticizes representatedness of sample if (1) stated intent in Introduction is true external validity, (2) stated intent is “applied” to specific sample, (3) conducting telephone or mail study

14. How do I get them to take part in my study? • Awareness • Advertising; announcements in your class; colleagues classes; flyers around campus; word-of-mouth; listservs; newsgroups; chatrooms; craigslist; social networking sites like Facebook, etc. • Incentivizing • Lottery; payment; extra-credit for class; give study the day before or after midterms; points system • Reduced Burdens • Make study (perceived as) short, simple, easy, fun, interesting • Multi-modal • Use all methods: in-person, online, mail, telephone • Email list • “Do you want to be contacted about future studies…” • There are companies from which you can buy email lists

15. How do I get them to take part in my study? • Using Psychology Principles • Authority • Culturally ingrained • Commitment/Consistency • Foot-in-the-door • Lowballing • Bait-and-Switch • Liking • “Halo effect”, • Targeted advertising • Reciprocation • Door-in-the-face • That’s-Not-All • Foot-in-mouth • Scarcity • Limited time • Valuable product • Social Proof • Emotion • Positive feelings (desire for positive self-esteem) • Guilt (and then provide way of eliminating the guilt) • Fear (and then provide way to prevent scary situation) • Appeals to self-interest • Personally relevant, or relevant to ingroup

16. How do I get them to take the entire study? • Why this is a problem • Some missing data sometimes means throwing out entire subject when analyzing the data • Those who drop may be different than those who don’t, and no real way to identify differences • Why there is no solution • If incentive (lottery, payment, points, etc), then by law everyone can receive the incentive whether or not they take part in the study, and/or whether or not they finish the study. • Can try to create more attractive study (e.g., short, simple, fun, easy, interesting, etc) but will always have some people who will not finish the study • Typically, response rate for _______ is ________. • Laboratory: excellent (subjects feel compelled) • Field study: average (attractive experimenter increases rate) • Online study: average (amount of lottery has no real effect) • Mail/telephone: very poor

17. Advanced Sources • Practical Sampling, by Henry, Sage Publications, 1990 (part of the Applied Social Research Methods Series)