Cromwell's Rule

Cromwell's rule, named by statistician Dennis Lindley, states that the use of prior probabilities of 0 ("the event will definitely not occur") or 1 ("the event will definitely occur") should be avoided, except when applied to statements that are logically true or false, such as 2+2 equalling 4 or 5.

The reference is to Oliver Cromwell, who wrote to the General Assembly of the Church of Scotland on 5 August 1650, including a phrase that has become well known and frequently quoted:

I beseech you, in the bowels of Christ, think it possible that you may be mistaken.

As Lindley puts it, assigning a probability should "leave a little probability for the moon being made of green cheese; it can be as small as 1 in a million, but have it there since otherwise an army of astronauts returning with samples of the said cheese will leave you unmoved." Similarly, in assessing the likelihood that tossing a coin will result in either a head or a tail facing upwards, there is a possibility, albeit remote, that the coin will land on its edge and remain in that position.

If the prior probability assigned to a hypothesis is 0 or 1, then, by Bayes' theorem, the posterior probability (probability of the hypothesis, given the evidence) is forced to be 0 or 1 as well; no evidence, no matter how strong, could have any influence.

A strengthened version of Cromwell's rule, applying also to statements of arithmetic and logic, alters the first rule of probability, or the convexity rule, 0 ≤ Pr(A) ≤ 1, to 0 < Pr(A) < 1.

An example of Bayesian divergence of opinion is in Appendix A of Sharon Bertsch McGrayne's 2011 book The Theory That Would Not Die: How Bayes' Rule Cracked The Enigma Code, Hunted Down Russian Submarines, & Emerged Triumphant from Two Centuries of Controversy. In McGrayne's example (suggested by Albert Mandansky), Tim and Susan disagree as to whether a stranger who has two fair coins and one unfair coin (one with heads on both sides) has tossed one of the two fair coins or the unfair one; the stranger has tossed one of his coins three times and it has come up heads each time. Tim judges that the stranger picked the coin randomly, i.e., assumes a prior probability distribution in which each coin had a 1/3 chance of being the one picked. Applying Bayesian inference, Tim then calculates an 80% probability that the result of three consecutive heads was achieved by using the unfair coin. Susan assumes the stranger either chose the unfair coin (in which case the prior probability the tossed coin is the unfair coin is one) or chose one of the other coins (in which case the prior probability the tossed coin is the unfair one is zero). Consequently, Susan calculates the probability that three (or any number of consecutive heads) were tossed with the unfair coin must be one or zero; if still more heads are thrown, Susan gains no more certainty that the unfair coin was picked than she had after the first head; Tim and Susan's probabilities do not converge.

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