Experimental Designs

Campbell & Stanley’s (1963) three general types of experimental designs Pre-experiments Quasi-experiments Full or true experiments

Key concepts Random assignment: assigning participants to the various conditions in an experiment to achieve equivalent groups for comparison purposes. Treatment condition: administering the experimental manipulation Control condition: no experimental manipulation or an alternative, benign manipulation Pretest: measurement of the dependent variable(s) prior to exposure to a treatment or control condition Posttest: measurement of the dependent variable(s) following the treatment or control conditions.

notation for experimental designs R = random assignment N = no random assignment 01 = pre-test (measurement) 02 = post-test (measurement) X1 = treatment, stimulus condition, or manipulation X0 = control group (no treatment)

Pre-experimental designs one-shot case study: A group (usually intact) is administered a treatment and then measured or observed. No attempt is made to randomly assign subjects to condition, nor does the design provide for any additional groups as comparisons. N X1 O1 one group, pre-test, post test design N O1 X O2 static group comparison: only one of two intact groups is given the experimental treatment. At the end of the treatment, both groups are observed or measured to see if there is a difference between them as a result of the treatment. X1 01 N intact groups, no pretest N X0 01

Quasi-experimental designs Pre-test, post-test, quasi-equivalent groups design N01 X O2 N01 O2 Times-Series design N01 02 03 04

Full Experimental Designs pre-test/post-test equivalent groups design R 01 X1 O2 01 O2 post-test only equivalent groups design R X1 O1 O1 Solomon four-group design R 01 X1 O2 02 O2 X1 O2 O2 a.k.a. two-group posttest only design

true/full experimental designs Solomon Four Group design: attempts to control for the possible "sensitizing" effects of the pre-test or measurement by adding two groups who have not been a part of the pre-test or pre-measurement process. R 01 X1 O2 02 O2 X1 O2 O2

Factorial designs (may be full or quasi-experimental) Advantage: Allows the researcher to uncover interaction effects 2 X 2 design Two independent variables Two levels/values per variable Four conditions being compared 3 X 2 X 2 design Three independent variables Three levels/values for one independent variable, two levels/values for the other two independent variables Twelve conditions being compared