Experimental research designs are founded on the assumption that the world works according to causal laws. These laws are essentially linear, though complicated and interactive. The goal of experimental research is to establish these cause-and-effect laws by isolating causal variables.

A softer view of the philosophical assumptions behind experimental designs is that SOMETIMES and IN SOME WAYS, the world works according to causal laws. Such cause-and-effect relationships may not be a final view of reality, but demonstrating cause and effect is useful in some circumstances.

Both of these views agree that some (if not all) important psychological questions are questions about what causes what. Experimental research designs are the tools to use for these questions.

The GOAL OF EXPERIMENTAL RESEARCH METHODS is to establish cause-and-effect relationships between variables.

We hypothesize that the Independent Variable caused the changes in the Dependent Variable. However, these changes or effects may have been caused by many other factors or Alternative Hypotheses.

The PURPOSE, therefore, of experimental designs is to eliminate alterntive hypotheses. If we can successfully eliminate all alternative hypotheses, we can argue--by a process of elimination--that the Independent Variable is the cause.

Good experimental designs are those which eliminate more alternative hypotheses.

FOR EXAMPLE: Say I am testing whether a new form of psychotherapy is successful at improving mental health. I hypothesize that this psychotherapy is the cause of improved mental health in the research participants.

I will use an experimental design to eliminate all (or as many as possible) alternative hypotheses. If I can eliminate alternative explanations, I will be able to make the case that the psychotherapy was the cause of the improvements in the research participants.