of his results by some rules of thumb which he has difficulty putting into words even to himself. To exactly what set of cues he is responding it is difficult to say, for apparently no one has made an analysis of the exact difference between records which are judged positive and those which are judged negative or inconclusive.

For the experimenter the problem is different, since he is obliged to say exactly how he has arrived at an answer. His procedure, therefore, will usually be, as in the Indiana studies, to settle upon some rather obvious aspect of response that can be definitely measured, and then find out how well he can do at detection with this information about each response. Naturally, he is discarding a good deal of information while he does this, information about other features of the response which might be supplementary or superior to that which he is using. For example, in most GSR work only the maximum amplitude of the response is considered. It may well be that the duration of the response also has a meaning (beyond its correlation with amplitude). In fact, an unreported portion of the Indiana study indicated that this was true. The field operator might allow for this feature intuitively along with other characteristics such as latency, doubleness or singleness of response, rate of rise, etc. Not that the experimenter could not study these things, but he must take them one by one, test them singly and according to various rules of combination and weighting, a laborious and lengthy process. His hope, of course, is that in the long run he will be able to tell the field operator just what he should take into account to secure maximum reliability of decision. Certain suggestions can be offered a priori and some from experimental evidence.

The logic of the detection task imposes certain requirements. Basically the assignment is one of differentiating two conditions, truth telling and lying, on the basis of readings which are correlated with them ("prediction" in the statistical sense of the word). The operator or experimenter must proceed by finding, for a given S, the mean response to the critical and to the neutral questions. The alternative, of simply comparing critical responses of an S to those of other persons, will not be satisfactory because Ss differ in their level of responsiveness to any stimulus. By representing the responses to critical questions by R o and those to neutral questions by R n , we may say the first quantity to be considered is R c — R n . We must then decide on the basis of data which sign of the result is indicative of lying, if either. We might, under some circumstances, then determine from data how many persons will be correctly classified when the difference is of a certain degree. The number of detections would

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