Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!caip!topaz!uwvax!husc6!harvard!ut-sally!utastro!bill From: bill@utastro.UUCP (William H. Jefferys) Newsgroups: net.sci Subject: Re: Parapsychology: more on the decline effect. Message-ID: <981@utastro.UUCP> Date: Thu, 10-Jul-86 11:05:18 EDT Article-I.D.: utastro.981 Posted: Thu Jul 10 11:05:18 1986 Date-Received: Fri, 11-Jul-86 08:16:09 EDT Organization: U. Texas, Astronomy, Austin, TX Lines: 80 Topher Cooper says: >Bill Jefferys ({allegra,ihnp4}!{ut-sally,knew}!utastro!bill, >bill@astro.UTEXAS.EDU) has claimed as an explanation for the decline effect >in parapsychology (the tendency for the scoring rate to decrease over the >course of an experiment) the following: >>Not if you have selected for further study precisely those individuals >>who scored high initially, and if you include those earlier trials >>in the overall test results. >I admitted that such the error of inappropriately mixing screening with >testing trials might have occurred on rare occasions in the >parapsychological literature (as a chemist might admit that dirty test >tubes may have been the actual cause of some published results). I thought >it was *very* unlikely that any such flaw was the source of any claimed >evidence for the decline effect which had appeared in a refereed >parapsychology journal or other legitimate piece of parapsychological >literature, however. The error would have been too obvious in this case >to have been missed by the referees. Aha! I see the problem now. We have been talking at cross-purposes. It is my fault; I was trying to keep my response to Dave short, and I worded it very badly. I didn't mean to accuse experimenters of such an obvious mistake. But I agree that that is how my comment reads. I have to apologize to you. I had in mind a more subtle kind of unconscious "selection" that could take place after the trials began in earnest. There could be a "selection effect" of the following sort: Alcock cites a number of studies showing that initial success in this kind of experiment is much more likely to lead to belief by the subjects (and presumably by the experimenter) that nonchance effects are involved, than is initial performance at the chance level followed by increasing success. (The outcomes of the "random" results in some of these studies were, unknown to the subjects, manipulated by the experimenter). In any population of subjects that has been selected for further study, a certain fraction will (purely by chance) continue to score well for a while, while others will revert quickly to the chance level. (This assumes there are in fact no other unknown biases affecting the study). There are a number of factors working that will tend to keep the subjects who continue to score well in the study for more and more trials, while those who initially fall by the wayside will tend to drop out. One is the tendency of the experimenter to want to go with a winner. The more spectacular the initial success, the more time will be spent with such a subject. After all, the name of the game is to publish papers, and no experimenter likes publishing papers about his failures. Another is the tendency of the subject to drop out if he or she is not showing "paranormal abilities". The sooner such a subject's performance drops to the chance level, the less the reinforcement, and the more likely it is that he or she will drop out. This would happen even if the experimenter tried to keep all subjects in to the bitter end of a prescribed number of trials. The net result would be a set of short trials with subjects who scored at the chance level; a smaller set of longer trials with subjects who initially scored above chance but reverted to chance rather quickly; and a small set of subjects who initially scored spectacularly, who got a lot of attention and many trials, and who also reverted to chance levels. In other words, a "Decline Effect". The reason why one doesn't see an "Increase Effect" is that few if any studies would do extensive testing on subjects that start out poorly. The effect could be compounded if there are biases (e.g., unconscious cueing) which initially inflate scores, but which come under better control as attention is focussed on a high-scoring subject. Is this an unreasonable scenario? If so, why? (Topher, if you respond to this, please E-mail me a copy, as I will be at a meeting for a few days & our machine expires news in 3 days). -- Glend. I can call spirits from the vasty deep. Hot. Why, so can I, or so can any man; But will they come when you do call for them? -- Henry IV Pt. I, III, i, 53 Bill Jefferys 8-% Astronomy Dept, University of Texas, Austin TX 78712 (USnail) {allegra,ihnp4}!{ut-sally,noao}!utastro!bill (UUCP) bill@astro.UTEXAS.EDU. (Internet)