It is typically skeptics who use these terms in response to claims made by Psychics , Astrologers and other Paranormalists to have predicted an event, when the original prediction was vague, catch-all, or otherwise non-obvious.
Most predictions from such figures as Nostradamus and James Van Praagh are deliberately written in a such a vague and ambiguous way as to make interpretation nearly impossible before the event, rendering them useless as predictive tools. After the event has occurred, however, details are Shoehorned into the prediction by the psychics or their supporters using Selective Thinking - emphasize the "hits", ignore the "misses" - in order to lend credence to the prophecy and give the impression of an accurate "prediction". Inaccurate predictions are simply not mentioned.
Supporters generally feel that the problem lies not with the wording of the prediction, but with the interpretation - much the same argument used by supporters of religious texts - but this then elicits the question: "What is the point of a prediction that cannot be interpreted correctly before the event?"
Accusations of postdiction might be applicable if the prediction was:
- The prediction makes a non-specific claim. For example it predicts "a disaster" of some kind but not ''what'' it is. Such a prediction can be massaged to fit any number of events. Likewise a prediction that does not state dates or places, or allows itself a large ''window'' of possible dates can be made to hit many possibilities. A prophecy attributed to Saint Malachy (but widely regarded as a 16th Century Forgery ) claims to predict the Succession Of Popes by giving each a brief description to each of them. However each description is suitably vague that it can be massaged to fit after the fact.
- The prediction has a very long cut-off date or none at all and therefore runs indefinitely. Many Nostradamus quatrains are open-ended and have been postdicted over the centuries to fit various contemporary events.
- The prediction is reused again and again in order to match the most "juicy" event. Nostradamus quatrains have been recycled numerous times.
- The prediction covers more than one possible outcome. For example, a Super Bowl prediction might hint that either side could win.
- The prediction is in fact many predictions, designed to cover a range of events and claim credit even if only one of them happens. For example, claiming that a particular date is "unlucky" and then citing a dozen or so things that might happen on it. See Selective Thinking .
- The prediction makes a claim for something that happens with enough frequency that a high hit rate is virtually assured. For example, predicting terrorism on any day of the year, or particularly around national holidays, anniversaries (or similar events), or religious festivals.
- The prediction makes a claim that is impossible to verify or Falsify . For example, a belief arose amongst a few in 2003 that a " Planet_X " would pass the Earth in May of that year. When it singularly failed to appear they claimed it was shrouded so that only an "educated eye" could see it and various other excuses, while discounting the most obvious reason - that Planet X does not exist at all in the form predicted.
- A prediction cannot be verified if there was no public record of when it was made. A famous example, was the psychic Tamara Rand who "predicted" that Ronald Reagan was in danger of someone with the initials "J.H". The video interview in which this prediction was made was shot the day after the assassination attempt.
- The prediction may be part of a series, but is singled out because it can be favourably interpreted, even if the series itself follows the laws of probability. For example, the prediction might correctly state movement on the stock market when previous or subsequent predictions have been wrong.
- . The postdiction resorts to tenuous allegorical explanations to turn literal misses into hits. For example the postdiction might explain that a famous person has suffered a 'spiritual' death to explain why they are still walking around despite a prediction that says otherwise.
- . The event must be "shoehorned" to fit the prediction because it differs in some significant way. For example, the prediction predicts an earthquake on one day when in fact it happens on a different day. Once again, Nostradamus supporters occasionally use this technique, as Nostradamus supposedly predicted the founding of the Institut Pasteur in 1888 (it was actually a year later) and the September 11 Terrorist Attacks on the 45th parallel (actually significantly southwards).
These types are not exclusive, so a prediction could be vague, statistically likely and open ended at the same time.
Accusations of postdiction can be avoided if the claimant follows some simple guidelines:
# Make one prediction per event.
# Structure the prediction so there is a brief summary, a detailed description, followed by any notes.
# Make the summary and description as unambiguous and specific as possible - state the nature of the event, the date, the location and other information plainly and clearly.
# Use plain language, not verse, allegory, flowery or other incoherent or non-obvious language. A prediction should not have to be "interpreted" - it should be self evident.
# If there is uncertainty about any details such as the location or date, state it clearly in the notes.
# Post all predictions to a public location such as a Usenet group or a mailing list which has a large subscriber base and a date stamped archive of posts.
# Don't massage the facts to fit the prediction.
# Acknowledge the misses along with the hits.
It may be that the claimant has reasons for not giving too much detail before the event. In that situation, the claimant should still post some detail but make the rest unreadable using an encryption tool such as GPG . The encrypted block can be attached with a summarised prediction, and a note that states on which day the key will be released to unlock the message. Failure to release the key on the stated date would be "counting the hits and not the misses" and would make the claimant liable to accusations of postdiction.
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