POLSCI 2NN3 Lecture Notes - Lecture 11: Random Assignment, Selection Bias, External Validity

• School: McMaster University
• Department: Political Science
• Course: POLSCI 2NN3
• Professor: Todd Alway

Document Summary

Scores between the control group and the experimental group on the dependent variable can be compared with any difference being attributed to the independent variable. Using a control group can increase the internal validity of an experiment. Threats to the internal validity of an experiment continued: selection bias. Control groups only form a control when they are effectively equivalent to the experimental group. Selection bias (in this context) implies that the control and experimental group are not equivalent. If the two groups are not equivalent, then any difference in scores between groups in the post-test: may be re ecting the initial difference between the groups, not the effect of the independent variable that we are interested in. Individuals in the sample are assigned to the control or the experimental group on the basis of random chance alone. As long as the groups are of a suf cient size, random assignment should equalize their average characteristics.