May 22, 2008

Another Bias on Deja Evaluation

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The problem of an inadequate and restricted sample is particularly disappointing in Neppe’s (1983e) book, one of the most extensively documented survey projects on deja vu. Although he included five groups—people with schizophrenia, people with temporal lobe epilepsy (TLEs), people with non-temporal lobe epilepsy, paranormal experients (people who believe in the paranormal), and controls—his control group consisted of 10 people who did not believe in paranormal phenomena (i.e., paranormal nonexperients).

Only 5 of these 10 controls had ever had a deja vu experience; thus, an extensive series of descriptive statistics and analyses was based on 5 persons. Though Neppe (1983e) admitted that data from this control group were not generalizable, it is unfortunate that such a great effort was expended on presenting the results from so few individuals.

Another bias is created by the context of the deja vu question, which is often embedded among items evaluating paranormal phenomena such as out-of-body experiences, belief in poltergeists, psychokinesis, channeling, spirit possession, thought control, telekinesis, hauntings, past-life experiences, lucid dreams, ESP, and hallucinations (Gaynard, 1992; Green, 1966; Greyson, 1977; Kohr, 1980; McClenon, 1988; Palmer, 1979; Ross & Joshi, 1992).

Note, however, that Ross and Joshi (1992) removed the deja vu question from their analysis of 15 other questions on the paranormal because the reported deja vu incidence was too high to be considered paranormal.

A particularly unfortunate example of this bias is in the General Social Survey by the National Opinion Research Center (NORC) at the University of Chicago. The NORC uses exceptional sampling procedures in their surveys, yet accompany the deja vu question with four others evaluating parapsychological phenomena, namely ESP, clairvoyance, contact with the dead, and out-of-body experiences.

Similarly, a Gallup poll (Gallup & Newport, 1991) positioned the deja vu item in a set of questions about belief in the following realms of the paranormal: ESP, astrology, ghosts, clairvoyance, witches, and devils. Although Gallup and Newport (1991) acknowledged that most psychologists do not consider deja vu to be a psychic or paranormal phenomenon, they clearly implied this to their survey participants.