Single and Repeated Observations
In theory, it would that experimenter’s job would be least complicated if he could always work with the same subjects, thus coming to know their characteristics and controlling for their past experiences. While this often proves to be possible, there are some experiments which, by their very nature, can only be carried out once on a given subject. For example, in his studies of double alternation behavior, Hunter found that once the human subject came upon the solution to the problem, the experiment was over and the subject could not be used again.
If is the case that a subject can be tested but once, the experimenter is then forced into the use of different subjects each value of his independent variable. If a great many subjects can be included in each such group, random assignment of subjects to groups may balance individual differences and the groups may be directly compared. Where such large numbers are not feasible, subject may be matched on the basis of a significant variable, and then so assigned that the groups are balanced before the crucial phase of the procedure is introduced. Finally, there are even some statistical techniques ( such as analysis of covariance ) that allow the experimenter to estimate the significance of his procedure in spite of having nonmatched groups with which to work. These techniques are discussed in many textbooks of statistics and experimental design.