COMM 88 Lecture Notes - Lecture 12: Random Assignment
Comm 88 Lecture 12
May 15, 2018
Experimental Research
•True experiments (X: IV, Y: DV, R: random assignment)
•Posttest only control group design
•R X1 Y (group 1)
•R X2 Y (group 2)
•Example
•R X1 (anti-smoking ad) Y (beliefs about smoking)
•R X2 (no anti-smoking ad) Y (beliefs about smoking)
•If get difference between group means on Y, the IV caused it
•Variations: more groups, several different treatments
•Pretest-posttest control group design
•R Y1 X1 Y2 (group 1)
•R Y1 X2 Y2 (group 2)
•Example:
•R Y1 (belief about smoking) X1 (anti-smoking ad) Y2 (belief about smoking)
•R Y1 (belief about smoking) X2 (no anti-smoking ad) Y2 (belief about smoking)
•Again, if you get difference between group means (on Y2), the IV caused it
•Possible problem: differences on Y2 might be the result of interaction of manipulation with
pretest
•Solomon four-group design
•R Y1 X1 Y2 (group 1)
•R Y1 X2 Y2 (group 2)
•R X1 Y (group 3)
•R X2 Y (group 4)
•Pretesting: should you or shouldn’t you
•Useful:
•To “check” on random assignment
•To get information on change
•But:
•Not necessary to establish causality
•Bad idea if treatment/pretest interaction is likely
•Example study: music and learning
•Hypothesis: listening to music (while studying) hinders learning (or improves learning) -
causal
•IV: music, DV: studying
•R X1 (music) Y (test score) group 1 → M = 67
•R X2 (no music) Y (test score) group 2 → M = 79
•Now what if we want to test for effects of another IV? Such as study task
Factorial designs
•Purpose: to examine the effects of 2 or more IVs simultaneously
•“Factors” are IVs - each factor/IV has at least 2 levels (conditions)
•Example:
•Music factor: music vs. no music AND study task factor: math problems vs. reading
comprehension
•A 2 X 2 design (2 levels of music X 2 levels of task)
