Maturation

Natural changes occur to persons over time. In experimental design, any natural change that could affect the experiment is known as maturation.   So, in addition to protecting against the misleading effects of history, an experimenter has to make sure that changes in the dependent variable are not due to naturally occurring changes among the subjects.   In all truthfulness, the results in our illustrative experiment would probably not be influenced by maturation.   The time period would be too short for naturally occurring changes to show up.   Maturation, however, can be a problem in experiments lasting months or longer.

Testing effect

Sometimes simply giving a pretest can influence the attitudes and behavior of subjects.   Questions included in the pretest, for example, could cause some people to begin thinking about prejudice in ways they had not done before.   As a result, when they were asked about the group in the posttest they might give less prejudiced views.   Consequently, the decline could have been at least partly due the pretest experience and not wholly because of the educational program.   Anytime a pretest is used there is danger of creating a testing effect.   Our experimental design also failed to protect against this threat to drawing a valid conclusion from the decline in prejudice.

Instrumentation

To get around the problem of testing effects, an experimenter can use different measuring instruments for the pretest and posttest measurements.   Changing measuring instruments, however, raises the question of the comparability of the instruments, or what is known as instrumentation.   If the instruments differ in their validity, reliability or degree of precision, differences in pretest and posttest measurements could be due to this fact and not because of changes in characteristics of the subjects.

Subject reactivity

In our experiment subject reactivity could have occurred.   The persons selected to participate in the experiment could have been pleased to be singled out for special attention and, as a result, express less prejudiced views.   Also, some might have guessed what the purpose of the educational program was and expressed views in keeping with this objective, even though their views had not changed that much.   Thus, subject reactivity and not the educational program could have at least partially accounted for the decline in the posttest measure of prejudice.

Experimental mortality

We do not mean to imply that experimenters are prone to early death or that subjects run some risk to their lives.   Research use of this term is far less menacing.   Experimental mortality refers to the loss of subjects during the life of an experiment.   As experiments extend over longer periods of time, some subjects are usually lost: Some move away, some become ill and cannot participate, and some just quit.    A high loss of subjects can distort the results of an experiment.

Experimenter bias

The investigator can be the cause of invalid conclusions as well.    After all, the experimenter develops the hypothesis, creates the conditions under which the experiment is carried out, and collects and analyses the data.   The investigator, therefore, is in a strong position to influence the results obtained from an experiment. Experimenters have to guard against this temptation.   Because of the powerful position of the experimenter, other researchers expect to see a detailed description of the experimental design used and a full display of the data obtained in the report of an experiment.

Returning to the example involving prejudice, you may now see some of the reasons why we could not say that the decline in prejudice was caused by the independent variable. A number of other factors could have caused the decline.     In experimental terms, we failed to meet the criterion of internal validity.