A bias caused by inappropriate averaging in experiments with randomized stimuli
2006 (English)In: Journal of Parapsychology, ISSN 0022-3387, Vol. 70, no 2, 233-254 p.Article in journal (Refereed) Published
Using simulations, a bias caused by inappropriate averaging in experiments with randomized stimuli is described. As an illustrating example, experiments aimed at demonstrating "presentiment" by showing arousal to be higher prior to arousing stimuli than prior to calm stimuli are considered. It is shown that such results could be obtained if (a) the participant believes that the likelihood of an activating stimulus being presented on the next trial increases after a calm stimulus has been presented (the gambler´s fallacy) and (b) overall arousal means are calculated across individual arousal means or for a pooled sample of all possible stimulus sequences. The effect becomes very small when participants are pooled before averaging, provided that the number of trials per participant and/or the number of participants is sufficiently large. The bias decreases as the length of the stimulus sequence increases, and becomes zero in an infinitely long sequence. Basically, the bias can be explained by the fact that the expected mean arousal level before calm stimuli increases as a function of sequence length. Various possible strategies for reducing or eliminating the bias are discussed, but none of them is judged to be fully satisfactory. A generalized version of the bias is outlined and discussed. It is argued that, in some form, the bias may occur in various types of experiments, both within and outside parapsychology. It is also argued that numerous previous experiments need to be checked for occurrence of the bias.
Place, publisher, year, edition, pages
2006. Vol. 70, no 2, 233-254 p.
prejudices, parapsychology, gamblers' fallacy
IdentifiersURN: urn:nbn:se:su:diva-16634OAI: oai:DiVA.org:su-16634DiVA: diva2:183154