Experimental Psychology PSY 433 Chapter 3 Experiments Designs

Experimental Psychology PSY 433 Chapter 3 Experiments -- Designs

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

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

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

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

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

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

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

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

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

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

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

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

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

Main Effects n There is one potential main effect for each IV A 2 X 8 design has two possible main effects n A 2 X 4 has three possible main effects n n 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).

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

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

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

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

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

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

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