Russel-Research-Method-in-Anthropology

118 Chapter 5
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.
Controlling for Threats to Validity
In what follows, I want to show you how the power of experimental logic
is applied to real research problems. The major experimental designs are
shown in figure 5.1. The notation is pretty standard. X stands for some intervention—a
stimulus or a treatment. R means that participants are randomly
assigned to experimental conditions—either to the intervention group that gets
the treatment, or to the control group that doesn’t. Several designs include
random assignment and several don’t. O stands for ‘‘observation.’’ O 1 means
that some observation is made at time 1, O 2 means that some observation is
made at time 2, and so on.
‘‘Observation’’ means ‘‘measurement of some dependent variable,’’ but as
you already know, the idea of measurement is pretty broad. It can be taking
someone’s temperature or testing their reading skill. It can also be just writing
down whether they were successful at hunting game that day.
The Classic Two-Group Pretest-Posttest Design with Random
Assignment
Figure 5.1a shows the classic experimental design: the two-group pretestposttest
with random assignment. From a population of potential participants,
some participants have been assigned randomly to a treatment group and a
control group. Read across the top row of the figure. An observation (measurement)
of some dependent variable or variables is made at time 1 on the members
of group 1. That is O 1 . Then an intervention is made (the group is exposed
to some treatment, X). Then, another observation is made at time 2. That is
O 2 .
Now look at the second row of figure 5.1a. A second group of people are
observed, also at time 1. Measurements are made of the same dependent variable(s)
that were made for the first group. The observation is labeled O 3. There
is no X on this row, which means that no intervention is made on this group
of people. They remain unexposed to the treatment or intervention in the