Russel-Research-Method-in-Anthropology

Research Design: Experiments and Experimental Thinking 117
ple who are victims of this treatment are more likely to have unstable marriages
anyway, even if they never experience violence.
Question: Do migrants to cities from villages in developing nations engage
in more entrepreneurial activities than stay-at-homes? If we could assign rural
people randomly to the treatment group (those engaging in urban migration),
we’d have a better chance of finding out. But we can’t, so selection is a threat
to the internal validity of the experiment. Suppose that the answer to the question
at the top of this paragraph were ‘‘yes.’’ We still don’t know the direction
of the causal arrow: Does the treatment (migration) cause the outcome (greater
entrepreneurial activity)? Or does having an entrepreneurial personality cause
migration?
6. 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. We’ll talk more
about those in chapter 10.
7. 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 some time ago, 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