Russel-Research-Method-in-Anthropology

114 Chapter 5
Kinds of Confounds: Threats to Validity
It’s pointless to ask questions about external validity until you establish
internal validity. In a series of influential publications, Donald Campbell and
his colleagues identified the threats to internal validity of experiments (see
Campbell 1957, 1979; Campbell and Stanley 1966; Cook and Campbell
1979). Here are seven of the most important confounds:
1. History
The history confound refers to any independent variable, other than the
treatment, that (1) occurs between the pretest and the posttest in an experiment
and (2) affects the experimental groups differently. Suppose you are doing a
laboratory experiment, with two groups (experimental and control) and there
is a power failure in the building. So long as the lights go out for both groups,
there is no problem. But if the lights go out for one group and not the other,
it’s difficult to tell whether it was the treatment or the power failure that causes
changes in the dependent variable.
In a laboratory experiment, history is controlled by isolating participants as
much as possible from outside influences. When we do experiments outside
the laboratory, it is almost impossible to keep new independent variables from
creeping in and confounding things.
Here’s an example of an experiment outside the lab. Suppose you run an
experiment to test whether monetary incentives help third graders do better in
arithmetic. Kids in the treatment classes get a penny for each right answer on
their tests; kids in the control classes get nothing. Now, right in the middle of
the school term, while you’re running this experiment, the Governor’s Task
Force on Education issues its long-awaited report, with a recommendation that
arithmetic skills be emphasized during the early school years. Furthermore, it
says, teachers whose classes make exceptional progress in this area should be
rewarded with 10% salary bonuses.
The governor accepts the recommendation and announces a request for a
special legislative appropriation. Elementary teachers all over the state start
paying extra attention to arithmetic skills. Even supposing that the students in
the treatment classes do better than those in the control classes in your experiment,
we can’t tell if the magnitude of the difference would have been greater
had this historical confound not occurred. That’s just the breaks of experimenting
in real life without being able to control everything.
2. Maturation
The maturation confound refers to the fact that people in any experiment
grow older or get more experienced while you are trying to conduct an experi-