How do quasi-experiments differ from "true" experiments?

• No random assignment, unable to hold conditions constant

When researchers use a quasi-experimental design they seek to compare the effects of a treatment condition to a control condition in which the treatment is not present-just like in a "true" experiment. However, in quasi-experiments, researchers often are unable to assign participants randomly to the conditions. In addition, the researcher may not be able to isolate the effects of the independent variable by holding conditions constant. Thus, participants' behavior (as measured by the dependent variable) may be affected by factors other than the independent variable.

Although quasi-experiments provide some information about variables, the cause-and-effect relationship (causal inference) may not be clear. The benefit of quasi-experimental designs, however, is that they provide information about variables in the real world. Often researchers conduct quasi-experiments with the goal of creating change. Psychologists have a social responsibility to apply what they know to improve people's lives; quasi-experiments help psychologists to meet this goal.

How do I conduct a quasi-experiment?