Main points

1. The model for the classical or true experiment is based on two equivalent groups, referred to as the experimental and control groups, each of which is measured on the dependent variable before the experiment begins (the pretest measurement); the experimental variable, also called the experimental treatment, is administered to the experimental but not to the control group; the dependent variable is measured again (the posttest) at the end of the experiment, and the results for the experimental and control groups are compared. It is expected that change in the dependent variable in the experimental group will be greater than that for the control group. This change is then attributed to the effect of the experimental variable.
2. Equivalent experimental and control groups can be created by randomization or by precision or frequency matching. Randomization is the preferred method for creating equivalent experimental and control groups.
3. Experiments are the strongest way to test hypotheses.
4. In conducting experiments, investigators have to guard against contamination. This is the accidental or uncontrolled exposure of the control group to the experimental treatment.
5. Quasi-experiments are based on some but not all the features of the classical experiment. Sometimes quasi- experiments are the closest approximation to conditions of the classical experiment that researchers can attain.
6. Internal validity refers to the extent that influences external to the experiment are minimized. External validity is the degree to which results of an experiment can be generalization to conditions of real life.
7. Quasi-experiments frequently do not meet the criteria for internal validity, but generally are higher than true experiments in terms of external validity. True experiments are high on internal validity, but often lack external validity.
8. Experimental designs can be varied by having several levels of experimental treatment with two or more experimental groups and an equivalent control group.
9. Strengths of the experimental method include control over conditions that can affect the dependent variable and the ability to apply the independent or experimental variable in controlled ways. Many experiments can also be carried out easily, quickly, and inexpensively by a single investigator. The main limitation of the experimental method is that conditions necessary to ensure internal validity can make it difficult to generalize results to the conditions of everyday life.