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The predictive value of a laboratory test is based on “prior probability,” the likelihood that the patient has a disease before testing has been performed.

“You just assign a prior probability to each of these models,” Kipping says.

So each hypothesis gets a prior probability of one half, much as if one were to flip a coin to decide a wager.

One fact is clear: if we assign a zero prior probability for such evidence coming our way, as some scientists did in the case of ‘Oumuamua by invoking the principle “it’s never aliens,” we will indeed never find any.

The problem is that researchers usually have no way of knowing what the prior probability is.