What are the two essential characteristics of an experiment? Why, and how, is each of these important for the internal validity of the experiment?
What will be an ideal response?
One essential characteristic of an experiment is that the researcher has control over
the experimental procedures. The researcher manipulates the variables of interest and
keeps all else uniform. That is, all participants should be treated exactly the same way-
-except for the manipulations that the experimenter is investigating. The other essential
characteristic of an experiment is that all participants are assigned randomly to the
different conditions. Because of random assignment to condition, at the beginning of
the study there should be no systematic differences between the groups of participants.
Internal validity is the degree to which there can be reasonable certainty that the
independent variables in an experiment caused the effects obtained on the dependent
variable. By having a great deal of control over the experimental procedures,
researchers can help ensure that the only differences between conditions are the
manipulations themselves and not some other factors. If other factors vary along with
the independent variables, then it is less clear that the independent variables are the
cause of any differences found in the dependent variable, thereby reducing the internal
validity of the experiment. Even if the researcher has a great deal of control and
ensures that the only differences in the treatment received by the participants across
conditions are the manipulations of the independent variables, it is possible that the
participants in one condition are different from the participants in other conditions in
important ways just by random chance. If this is the case, the differences found in the
dependent variable may be a function of these other differences rather than the result
of the manipulations. If the participants are randomly assigned to the conditions,
however, the chances that the participants differed in ways other than those created by
the manipulations become very improbable, especially with larger samples. Differences
found in the dependent variable can thus be attributed to the manipulations of the
independent variables rather than to pre-existing differences among the participants,
thereby creating internal validity.
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