Experimental Psychology PSY 433

1. Experimental PsychologyPSY 433 Chapter 3 Experiments -- Designs

2. Two Kinds of Influence on DV • Systematic – affects the DV in the same way each time. • Can introduce bias into results. • When it occurs due to the manipulation of the IV we call it an “effect.” • Random – varies and thus affects the DV differently from observation to observation. • Can introduce “noise” into results. • Typically makes it more difficult to observe systematic influences.

3. Confounding Variable • Any variable, besides the IV, that can influence the DV. • A potential cause for the experimental effect, other than the IV. • An alternative explanation for observed findings in a study. • Any variable whose values change systematically across levels of the IV.

4. Control Variable • A variable whose values remain the same across levels of the IV (e.g., room temp, light levels, time-of-day, etc). • A goal in experimentation is to control as many variables as possible, to eliminate their potential effect on the DV. • Eliminates both confounds and noise. • Except for the IV, each subject should have as closely similar an experience in the experiment as possible.

5. Random Variable • Variable whose values vary randomly in an unbiased way across levels of the IV. • Random variables are usually created by the process of random assignment to levels in the experiment.

6. Subject Variable • A personal characteristic • Sex, height, weight, age, education, ethnicity, socio-economic status, etc. • Sometimes called a “demographic” variable. • Data describing subject characteristics should be collected in every study. • Can be controlled or left to vary unsystematically (through random assignment to levels).

7. Control Group • The group that receives no intervention, “zero” or “the absence of” the IV. • Eg, the placebo group in a drug experiment. • The group that serves as a baseline to compare with the behavior of the experimental groups.

8. Experimental Groups (Levels) • The groups that receive non-zero values of the IV, the groups receiving some intervention. • The drug groups in a drug study. • Also called “levels” of the IV. • The performances of these groups are compared with the performance of the control group.

9. Conceptual Definition • Definition of a variable at the conceptual or idea level. • Tends not to be very precise, more general. • May be expressed in terms of constructs (intervening variables).

10. Operational Definition • Specifies the operations or procedures necessary to measure the variables relevant to the conceptual definition. • Very precise. • Not general or vague at all. • Tells how the dependent variable was measured. • There may be many operational definitions (and DVs) for a single conceptual definition.

11. ODs and CDs - Example 1 • Conceptual - Amount of alcohol consumed • Operational - # of beers in 1 hour (0,1,2,3) • Operational - grams of alcohol/kg body weight • Operational – blood alcohol content, BAC (mg alcohol/deciliter blood).

12. ODs and CDs - Example 2 • Conceptual - Helping behavior • Operational - # of people who help a “victim” in a defined context. • Operational - duration of helping behavior. • Operational - # seconds before helping occurs (latency).

13. Complex Designs • More than one IV • Left/Right and 1, 5, or 10 spaces from center • More efficient than single IV experiments • Gives more information • Allows analysis of main effects and interactions.

14. Complex Designs - Terminology • An IV is called a factor • number of numbers = how many IVs there are • values of numbers = how many levels each IV has • Examples: • “2 X 2 design” (two IVs, each with 2 levels) • “2 X 3 design” (first IV has 2 levels, second IV has 3 levels) • “2 X 8 design” (first IV has 2 levels, second IV has 8 levels) • “2 X 2 X 4 design” (first IV has 2 levels, second IV has 2 levels, third IV has 4 levels).

15. Main Effects • There is one potential main effect for each IV • A 2 X 8 design has two possible main effects • A 2 X 2 X 4 has three possible main effects • A main effect is present if an IV had a significant effect on the experiment’s outcome (regardless of the effects of the other IVs).

16. Interactions • Please memorize: “An interaction occurs if the effect of one IV varies depending on the level of the other IV”

17. Designing Experiments • Two general types of designs • Between-subjects (independent groups) = each group gets one level of the IV  • Within-subjects (repeated measures) = each subject gets all levels of the IV • Equivalency of groups at each level is built-in for within-subjects and achieved by random assignment for between-subjects • Within designs require fewer subjects because there is less variability between levels.

18. Order Effects • Order effects (practice effects) = experiencing one level affects behavior in another level • Effects of practice, boredom, fatigue • Example: Does content (biology text vs. novel) affect proofreading speed? Order is Biology-Novel • Order effects are controlled in within-subjects designs by randomizing or counterbalancing the presentation orders.

19. Differential Carryover Effects • Carryover effects, differential/asymmetrical transfer effects occur when experiencing one level affects performance on the next. • The effect of the first level on the second level differs depending on which comes first. • Effect of B following A ≠ effect of A following B • Confound occurs when one level consistently precedes the other.

20. (no practice) (practice) Group 1 Biology Novel 1 2 (no practice) (practice) Group 2 Novel Biology 1 2 Order Effects in Proofreading

21. (no practice) (practice) Group 1 Neutral instructions Special instructions 1 2 (no practice) (practice) Group 2 Special instructions Neutral instructions 1 2 Differential Carryover Effects in Problem Solving

22. Other Considerations • Mixed designs (some between, some within) • Small-n designs • Matched groups designs • Demand characteristics = cues that tell subjects how they should behave (eg, drug studies) • Blind and double-blind procedures • Internal and external validity • Quasi experiments.