Random assignment
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Experiments are powerful because of RA
.
RA also eliminates alternative explanations involving CVs.
Example: randomly assign people to caffeine/ no caffeine Because of RA, the 2 groups won't differ in sleep (or any CVs)
RA helps ensure that groups are similar before manipulation
Thus, any differences after the manipulation, are due to the manipulation, not CVs.
Increases confidence that the groups are "before the manipulation"
Thus, post-manipulated differences in DV should be due to manipulation
random assignment
randomly assigns participants to a condition