Research Design in Clinical Psychology

1. Research Design in Clinical Psychology Lecture 4 Group Designs (Chapter 6 in Kazdin)

2. Subject Selection • Random selection • Equal probability that all subjects within the population can be selected • Extent of random selection improves External Validity • Concerns when convenience sample is used • Otherwise issues of participant diversity is critical • Sometimes specific samples are chosen (e.g., particular gender or ethnic group only)

3. Assignment • Random: Subjects assigned across groups randomly • Strongest defense against threats to internal validity • Limits effect of nuisance variables, but can’t guarantee equivalence on all other variables • May not produce = sized groups • Matching: Guarantees equivalence on other key variables • Pairing based on pretreatment scores or blocked by ranking • Can do so on gender or other variable (including striation)

4. Pretest-posttest control group design R O1 X O2 R O4 O4 • Strengths • Allows for matching at onset using pretest scores • Reduced variance and increased power • Allows for better examination of change across groups • Allows for predictors and descriptors of attrition • Allows for examination of several possible confounds • Weaknesses • Testing effects • Pretest sensitization • Sooner vs later tradeoff

5. Posttest only control group design R X O1 R O2 • Flip strengths/weakness of pretest posttest design

6. Soloman Four-Group Design R O1 X O2 R O3 O4 R X O5 R O6 • Evaluates the effect of the pretest • Typically only used when really, really sure that pretesting may produce problems, but also is necessary to limit threats to internal validity.

7. Factorial Design • Allow for simultaneous investigation of the effect of two or more variables • Allows for testing of interactions

8. Quasi-Experimental Designs • Includes both pretest, posttest, and other variations nonR O1 X O2 nonR O3 O4

9. Multiple-treatment designs • May be called counterbalanced designs • Each tx is experienced by all subjects • The manner in which this is done determines the specific type of design

10. Crossover Design • Subjects differ only in order they receive tx • Switch at ½ point R O1 XA O2 XB O3 R O4 XB O5 XB O6

11. Multiple Treatment Counterbalanced designs • Can simply randomly determine sequences, but may produce oddities with small samples • Arranging sequences where each treatment occurs in a position only once is a latin square • Can evaluate effect of order as a function of • Groups (Rows) • Positions (Columns) • Treatments • Advantages of completely balanced design • Every order once and only once

12. Order Considerations • Order effects • Order of tx might affect results • Focus only on order and not the specific tx • Sequence effects • Arrangement of treatments contributes to effects • Aka multiple tx effects or carryover effects • For both, must consider ceiling and floor effects