Prevention of contamination requires special care in the design of experiments. In most developing countries, close communication is common among relatives, friends, and co-workers.    Consequently, it is difficult to carry out valid experiments when subjects are selected from the same village or other settings, such as a single office or factory, where the potential subjects know each other well.    Experimenters avoid this problem by selecting experimental subjects from one village or community and control subjects from another village or community similar to that of the experimental group. The experimental variable is then applied in the setting selected as the experimental site but not in the setting being used as the control site.    In each location subjects can be randomly selected for pretesting and posttesting.

This approach, however, has several dangers.   First, the experimenter has to be sure that the members of the two villages or sites used are alike in all relevant ways.   Second, there is still the danger of contamination as a result of communication between members of the two groups.   During the experiment, the experimenter has to maintain observation of the members of the two groups to see if communication does occur.    If it does, the control group may be contaminated which would destroy the conditions of the experiment.

In addition to contamination, other threats to the internal validity of an experiment can arise. Social Interaction Threats describes some of these. If you are planning to conduct an experiment, you will want to be aware of how human interaction among the participants in an experiment, such as program administrations and staff, the experimenter (yourself), and subjects, can interact in ways to destroy or greatly limit the internal validity of your experiment.

Another site, Experimental Design presents the basic ideas behind experimental design; it also includes links to descriptions of various experimental designs not covered in this chapter.  

Quasi-experimental designs

Quasi-experimental designs are based on some, but not all, of the characteristics of the classical experimental design. Some quasi-experimental designs are based on randomization, but lack a pretest measurement; others may be based on observations for only one group; all involve at least a posttest measurement. Also, in most quasi-experimental designs the investigator has relatively little control over the independent variable. Instead, the investigator creatively seeks situations where the at least some of the principles of experimental design can be applied.

The distinguishing feature of most quasi-experiments is that they are carried out in the natural conditions of everyday life.   Examples include the study of the effects of family planning programs, effects of supplemental food on the intellectual development of young children, and whether agricultural practices change as a result of   educational programs. Three more frequently used quasi- experimental designs are described next.

Nonequivalent control group design

This design is frequently necessary because subjects cannot be randomly assigned to the experimental and control groups. Under these conditions the best that can be done is to create a nonequivalent control group.

Researchers in Sudan wanted to see if they could teach mothers ways of stimulating the development of young children (Grotberg and Badri, 1986). Following a survey of families in several areas of Omdurman, Grotberg and Badri assigned families to either an experimental or a control group. Frequency matching was used to create roughly equivalent groups in terms of socio-economic status, which was known to be associated with the kind of child-rearing practices used by mothers. This eliminated the influence of this powerful variable. Before the experiment began, all children in both the experimental and control groups were extensively tested for developmental status.

The goal of the experiment was to teach mothers how to provide greater stimulation for their children, and, thereby, enhance the psychological and social development of their children. The experimental treatment, another term for the experimental variable, consisted of teaching sessions with mothers, based on the developmental status of each child. Sessions lasting one to two hours were conducted twice a week for nine weeks. Mothers and children in the control continued their interaction as before, with no training sessions. At the end of the experiment, all children in both groups were retested with the same set of psychological and social development tests.