Mortality Threats
As Trochim states, "Mortality doesn't mean that people in your study are dying (although if they are, it would be considered a mortality threat!)." Instead, a mortality threat means that participants drop out of the study between the pretest and the posttest, and an observed effect may be caused by the fact that the make-up of the group is not the same at both stages of measurement. For example, if you are evaluating a marriage counseling program, and a number of couples drop out, the effect you observe at the end of the program may be due to the fact that the couples who dropped out were those who were not willing to make an effort, while those who stayed really wanted to work their problems out. A positive effect may simply indicate that people who are willing to make an effort are more likely to resolve their marital problems.
How do you think a comparison group could alleviate this threat to internal validity?
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