one of which may be responsible for part or all of the observed performance increase (Campbell & Stanely, 1969). What this means is that there are a number of alternative variables, other than the training variable, that could have resulted in performance being increased. Another way of saying this is that there are several alternative explanations for any observed performance effects.
One possible alternative explanation for job performance increase lies in the fact that other changes in trainees' jobs may have been occurring at the same time as the training program. For example, employees may have received salary increments and as a result were more motivated to improve the quality of their job performance. These simultaneous changes may be partially or largely responsible for the change in performance measure--a phenomenon usually called "history effects."
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