Introduction

Seeking
causal
relationships

Alternative
explanations

Internal validity

The classical experiment

Quasi-
experimental designs

Quasi-
experimental designs

External validity

Further
variations
in experimental design

Strengths & limitations of experimental research

Aids

Chapter 9: Designing Experiments

Introduction

In its simplest form, an experiment consists of making changes in a variable and, under carefully controlled conditions, observing the effects of the change on some other variable. Experiments are especially valuable for testing hypotheses that suggest a cause and effect relationship. Clear understanding of a presumed causal relationship is essential for understanding the logic and design of experiments. Therefore, we begin by reviewing these conditions. Chapter 3 describes these conditions in more detail.

Seeking causal relationships

Necessary conditions

Three conditions are necessary for arguing that one variable is the cause of changes in another variable. These are:

1. The variable believed to be the cause must always precede its effects;
2. The two variables must be consistently and preferably strongly associated; and
3. No other explanation provides a more satisfactory explanation of the observed changes in the dependent variable than the one put forth as its cause.

In designing an experiment, the researcher has to create conditions that meet or that come as close as possible to meeting these requirements.

Application of conditions

An illustration will show the difficulty of meeting the conditions for establishing causality. Some persons have   negative or hostile   attitudes,   often   unfounded,   about   a particular group of persons.    Social psychologists refer to such views as prejudice.    Obviously, there are many causes of prejudice, and many conditions that could increase or reduce attitudes of prejudice toward some group. Suppose we wanted to conduct an experiment to see if we could reduce attitudes of prejudice among members of some group toward another group. Our first step would be to discover factors that are known to be related to feelings of prejudice.

Through a review of the literature we might discover that persons with high prejudice towards members of some group tend to report that they had heard only negative things about that group. Using this as our theoretical basis, we could hypothesize that providing positive information about the group in question will reduce prejudice toward that group. In this hypothesis, the providing positive information would be designated as the experimental or independent variable and prejudice would become the dependent variable.

Our next step would be to design an experiment to test this hypothesis.   We would start by getting some people, called subjects, to agree to participate in the experiment. Let's say we got 50 subjects to agree.   We also have to create the conditions under which the subjects experience the experimental variable.   Here is where creativity in design comes in. We could create whatever kind of experience we think would be most effective in reducing prejudice.