If the experiment shows that one condition was better than another, we know it probably wasn't because one condition had better studiers in it than the other. Sometimes,though, a subject can't be in all conditions. Perhaps after the jackhammer condition, subjects need too long a break to recover from temporary deafness to be of any use in the other conditions. Random Selection . Biased samples interfere with collecting accurate data on which to draw deductive conclusions.
Random selection is a technique to help reduce bias.
The important point is that selection of subjects or selection of stimulus materials be made so as not to bias the outcome of the results in favor of the alternative hypothesis. Instead, from a pool of words that fit the criteria, a sample is randomly selected that will serve as materials in the experiment. Similarly, if it is a memory experiment, the experimenter should not ask that only good rememberers sign up. It is difficult to know a priori what needs to be controlled. The experimenter intends to control the independent variables and the situation in which the experiment is conducted.