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Mortality

The mortality confound refers to the fact that people may not complete their participation in an experiment. Suppose we follow two sets of Mexican villagers—some who receive irrigation and some who do not—for 5 years. During the 1st year of the experiment, we have 200 villagers in each group. By the 5th year, 170 remain in the treatment group, and only 120 remain in the control group. One conclusion is that lack of irrigation caused those in the control group to leave their village at a faster rate than those in the treatment group.

But what about those 30 people in the treatment group who left? It could be that they moved to another community where they acquired even more irrigated land, or they may have abandoned farming altogether to become labor migrants. These two outcomes would affect the results of the experiment quite differently. Mortality can be a serious problem in natural experiments if it gets to be a large fraction of the group(s) under study.

Mortality also affects panel surveys. That’s where you interview the same people more than once to track something about their lives. (More about panel studies in chapter 9.)

Diffusion of Treatments

The diffusion of treatments threat to validity occurs when a control group cannot be prevented from receiving the treatment in an experiment. This is particularly likely in quasi-experiments where the independent variable is an information program.

In a project with which I was associated, a group of African Americans were given instruction on modifying their diet and exercise behavior to lower their blood pressure. Another group was randomly assigned from the population to act as controls—that is, they would not receive instruction. The evaluation team measured blood pressure in the treatment group and in the control group before the program was implemented. But when they went back after the program was completed, they found that control group members had also been changing their behavior. They had learned of the new diet and exercises from the members of the treatment group.

 
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