Experimental Design Experimental Psychology Confounding Variables A variable that varies along with the independent variable. Confounding occurs when the effects of the IV and an uncontrolled variable are intertwined so you cannot determine which of the variables is responsible for the observed effect. Reminder: Internal Validity Basic Experiments • Posttest‐only Design 1. Two or more equivalent groups of participants • Selection Effects & Random Assignment 2. Introduce the Independent Variable • Control vs. Experimental; Amount 3. Measure the effect of the IV on the DV. Basic Experiments • Pretest‐Posttest Design The only difference is the addition of a pretest before the manipulation of the IV to ensure equivalent groups. Basic Experiments • Advantages & Disadvantages of Each Design 1. Sample Size vs. Randomization 2. Pretest may be necessary to equate groups 3. Mortality 4. Awareness of pretest intent? Demand characteristics! • Solomon Four Group Design Assigning Participants • Independent Groups Design (Between Subjects) – Random assignment to different levels of IV – Requires many participants – Controls for unknowns Assigning Participants • Repeated Measures Design (Within Subjects) – All participants experience all levels of IV – Less participants – Controls Random Error…more likely to find statistically significant results… if they exist – Order Effects & Carryover Effects • Practice, Fatigue, etc. Assigning Participants • Repeated Measures Design (Within Subjects) – Counterbalancing • Different Orders of Treatment Conditions Assigning Participants • Repeated Measures Design (Within Subjects) – Latin Squares A technique to control for order effects without having all possible orders. A limited set of orders is constructed to ensure that (1) each condition appears at each ordinal position and (2) each condition precedes and follows each condition one time. Assigning Participants • Repeated Measures Design (Within Subjects) – Time Between Treatments/Levels/Phases • Matched Pairs Design: Ensures that the groups are equivalent (on the matching variable) prior to introduction of the IV. Conducting Experiments.