For example, in evaluating the effectiveness of a new therapy, the mean performance of the therapy in one treatment group might be compared to the mean effectiveness of a placebo (or a well-known therapy) in a second, control group. In this context, differences between individuals in their reaction to the experimental and control manipulations are actually treated as errors rather than as interesting phenomena to study. This approach is applied because psychological research depends upon statistical controls that are only defined upon groups of people.
for example in evaluating the effectiveness of a new therapy the mean performance of the therapy in one treatment group might be compared to the mean effectiveness of a placebo (or a well-known therapy) in a second control group in this context differences between individuals in their reaction to the experimental and control manipulations are actually treated as errors rather than as interesting phenomena to study this approach is applied because psychological research depends upon statistical controls that are only defined upon groups of people