A two-sample test

In certain investigations involving an experimental group and a control group, the effect of the experimental treatment may be expected to manifest itself by increasing the scores of some subjects but by decreasing the scores of other subjects. Examples arise in the area of defensive behavior. Customary tests of differences between means are inappropriate for assessing the existence of effect of the experimental treatment in such cases. A test based on the ranks of the observations is proposed; it will be sensitive to extreme scores (either large or small or both) for experimentals.