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Meta-analysls of Forced-Choice Precognition Experiments
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OBJECTIVES
Precognition refers to the noninferential prediction of future events.
Anecdotal claims of “future knowing” have occurred throughout human
history, in virtually every culture and period.
Today, such claims are
generally believed to be based on factors such as delusion, irrationality, and
superstitious thinking.
The concept of precognition runs counter to
accepted notions of causality and appears to conflict with current scientific
theory.
Nevertheless, over the past half-century, a substantial number of
experiments have been reported by more than 60 investigators claiming
empirical support for the hypothesis of precognition. Subjects in
forced-choice experiments, according to many reports, have correctly
predicted to a statistically significant degree the identity (or order) of target
stimuli randomly selected at a later time.
We performed a meta-analysis of forced-choice precognition
experiments published in the English-language research literature between
1935 and 1987.
Five major questions were addressed through
this
meta-analysis:
•
Is there overall evidence for accurate target identification
(above chance hitting) in experimental precognition
studies?
•
Is there overall evidence that investigators can accurately
predict tendencies toward hitting and missing?
• What
is the magnitude of the overall
(directional and
predicted) precognition effect?
•
Is the observed effect related to variations in methodologi-
cal quality that could allow a more conventional
explanation?
• Does precognition performance vary systematically with
potential moderating variables, such as differences in sub-
ject populations, stimulus conditions, experimental setting,
knowledge of results, and time interval between subject
response and target generation?
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Meta-anafysls of F^ceo-Choice Precognition Experiments
DELINEATING THE DOMAIN
Source of Studies
Parapsychological research is still academically taboo and it is unlikely
that there have been many dissertations and theses in this area that have
,
escaped publication.
Our retrieval of studies for this meta-analysis
is
therefore based on the published
literature.
The studies include
all
forced-choice precognition experiments appearing in the peer-reviewed
English-language parapsychology journals:
Journal of Parapsychology,
Journal (and Proceedings) ofthe SocietyforPsychical Research, Journalofthe
American SocietyforPsychicalResearch, European JournalofParapsychology
(including the Research Letter of the Utrecht University Parapsychology
Laboratory), and Research in Parapsychology.
Criteria for Inclusion
Our review is restricted to fixed length studies in which significance levels
and effect sizes based on direct hitting can be calculated.
Studies using
outcome variables other than direct hitting, such as run-score variance and
displacement
effects, are included only
if the report provides relevant
information on direct hits (i.e., number of trials, hits, and probability of a
hit).
Finally, we exclude studies conducted by two investigators, S. G. Soal
and Walter J. Levy, whose work has been unreliable.
Many published reports contain more than one experiment or
experimental unit.
Experiments involving multiple conditions are treated
as separate study units.
Outcome Measures
Significance Levels: We calculated two significance estimates for each
study. The directionalz-score (zdir) measures the subjects’ success in scoring
in the direction of their intention. Thepredictedz-score (zpred) measures the
investigator’s success in predicting the relative strength or direction of the
outcome through conditional comparisons, experimental manipulations, or
correlations; above chance scoring (hitting) is assumed in single condition
experiments unless psi-missing is explicitly predicted. Predicted z’s have
Psychophysical Research Laboratories
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Meta-analysis of Forced-Choice Precognition Experiments
positive signs if the study outcome supports the investigator’s hypotheses
and have negative signs
if the outcome
is opposite the investigator’s
hypotheses. The use of these two measures allows us to assess both overall
accuracy (hitting) and lawfulness (predictability).
Effect Sizes: Most parapsychological experiments, particularly those in
the older literature, have used the
trial rather than the subject as the
sampling unit. Thus, we must use a trial-based estimator of effect size. The
effect size for each study is thez-score divided the square root of the number
of trials in the study. As with significance levels, we have two effect sizes for
each study. One reflects overall directional hitting (ESdir) and the other is
based on the investigators’ predictions of hitting or missing (ESpted).
General Characteristics of the Domain
We located 309 studies in 113 separate publications. These studies were
contributed by 62 different senior authors and were published over a 52-year
period, between 1935 and 1987.
Considering the half-century time-span
over which the precognition experiments were conducted, it is not surprising
that the studies are quite diverse.
The data base comprises nearly 2 million individual trials and more than
50,000 subjects.
Study sample
sizes range from 25 to 297,060
trials
(median =
1,194).
The number of subjects ranges from
1 to 29,706
(median =
16).
The studies employ a variety of methodologies, ranging
from guessing Zener cards and other card symbols, to automated random
number generator experiments (Figure
1).
The domain encompasses
diverse subject populations: the most frequently used population is students
(used in approximately 40% of the studies); the
least frequently used
populations are the experimenters themselves and animals (each used in
about 5% of the studies).
Though a few studies tested subjects through the mail, more typically
subjects were tested in person, either individually or in groups.
Target
selection methods range from manual card-shuffling or dice-throwing to the
use of random number tables or random number generators.
The
time-interval between the subjects’ responses and target generation varies
from less than one second to one year.
Psychophysical Research Laboratories
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Meta-analysis of Forced-Choice Precognition Experiments
Psychophysical Research Laboratories
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