The Uniqueness of Experimental Research - Experimental research is unique in that it is the only type of research that directly attempts to influence a particular variable, and it is the only type that, when used properly, can really test hypotheses about cause-and-effect relationships. Experimental designs are some of the strongest available for educational researchers to use in determining cause and effect.
Essential Characteristics of Experimental Research - Experiments differ from other types of research in two basic ways ― comparison of treatments and the direct manipulation of one or more independent variables by the researcher.
Randomization - Random assignment is an important ingredient in the best kinds of experiments. It means that every individual who is participating in the experiment has an equal chance of being assigned to any of the experimental or control conditions that are being compared.
Control of Extraneous Variables - The researcher in an experimental study has an opportunity to exercise far more control than in most other forms of research.
- Some of the most common ways to control for the possibility of differential subject characteristics (in the various groups being compared) are randomization, holding certain variables constant, building the variable into the design, matching, using subjects as their own controls, and the statistical technique of ANCOVA.
Weak Experimental Designs - Three weak designs that are occasionally used in experimental research are the one-shot case study design, the one-group pretest-posttest design, and the static-group design. They are considered weak because they do not have built in controls for threats to internal validity.
- In a one-shot case study, a single group is exposed to a treatment or event, and its effects assessed.
- In the one-group pretest-posttest design, a single group is measured or observed both before and after exposure to a treatment.
- In the static-group comparison design, two intact groups receive different treatments.
True Experimental Designs - Several stronger designs that are more commonly used are true experimental designs, matching designs, counterbalanced designs, time-series designs, and factorial designs. These designs do have at least some controls built into the design to control for threats to internal validity.
- The randomized posttest-only control group design involves two groups formed by random assignment and receiving different treatments.
- The randomized pretest-posttest control group design differs from the randomized posttest-only control group only in the use of a pretest.
- The randomized Solomon four-group design involves random assignment of subjects to four groups, with two being pretested and two not.
Matching - To increase the likelihood that groups of subjects will be equivalent, pairs of subjects may be matched on certain variables. The members of the matched groups are then assigned to the experimental and control groups.
- Matching may be either mechanical or statistical.
- Mechanical matching is a process of pairing two persons whose scores on a particular variable are similar.
- Two difficulties with mechanical matching are that it is very difficult to match on more than two or three variables, and that in order to match, some subjects must be eliminated from the study, since no matches can be found.
- Statistical matching does not necessitate a loss of subjects.
Quasi-Experimental Designs - In a counterbalanced design, all groups are exposed to all treatments, but in a different order.
- A time-series design involves repeated measurements or observations over time, both before and after treatment.
Factorial Designs - Factorial designs extend the number of relationships that may be examined in an experimental study.
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