POLSCI 3N06 Lecture Notes - Random Assignment, Internal Validity, Ecological Validity
23 Nov 2013
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Political science 3n06 2013 lecture 5b: experimental methods. Must show them an ad (but not an attack ad) Introduce a control group. countered by using a control group. Scores between the control group and the experimental group can be compared with the 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: how can you ensure that groups start out the same, 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 so, then the difference in scores in the post-test (between groups) may be reflecting the difference in subjects, not the effect of the independent variable that we are interested in. Ways to enhance pre-test group equivalence: probability sampling.
