## Why probability cannot be subjective

A reader recently disputed my condensation of the tenets of *Bayesian subjective probability*. (I promised a thread on which we could discuss the matter more fully, so here it is.) Here is what I said:

To [subjective Bayesians], all probabilities are experiences, feelings that give rise to numbers which are the results of bets you make with yourself or against Mother Nature (nobody makes bets with God anymore). To get the probability of A you poll your inner self, first wondering how you’d feel if A were true, then how you’d feel if A were false. The sort of ratio, or cut point, where you would feel equally good or bad becomes the probability. Subjective Bayesianism, then, was a perfect philosophy of probability for the twentieth century. It spread like mad starting in the late 1970s and still holds sway today; it is even gaining ground on frequentism. In its favor, it should be noted that, after we get past the bare axioms, the math of subjective Bayesianism and logical probability is the same.

The formal name subjectivists have given to guessing probabilities is *elicitation*, i.e. the process whereby you “poll your inner self.” I invite all to search this term, where you will find many sources. A good summary is given by this paper from the Aviation Human Factors Division Institute of Aviation at the University of Illinois at Urbana-Champaign:

Gamble Methods [of probability elicitation]. Probabilities can also be determined using two gamble-like methods.

In the certain-equivalent method, the expert chooses either a certain payoff or a lottery where the payoff depends on the probability in question, and the elicitor adjusts the amount of the certain payoff until the expert is indifferent between the two choices. In the lottery-equivalent method, the expert chooses either a lottery where the outcome depends on a probability set by the elicitor or a lottery where the outcome depends on the probability in question.

Now, if that doesn’t sound like “To get the probability of A you poll your inner self, first wondering how you’d feel if A were true, then how you’d feel if A were false. The sort of ratio, or cut point, where you would feel equally good or bad becomes the probability” then I’ll eat my hat (the old straw one, the Montecristi, that no longer fits).

There are whole groups of people whose job is to investigate ways of guessing probabilities. One group is, I kid you not, BEEP (Bayesian Elicitation of Experts’ Probabilities) at the University of Sheffield. They have a wide array of, semi-psychological, semi-statistical papers on the pitfalls and joys of probability guessing. An excellent summary is this paper by some pretty big wigs in statistics.

**Back of the envelope**

As rough estimates, and in many cases, I have absolutely no problem with guessing. I’d even go so far as to say that when any decision has to be made, then unless the situation can be completely deduced, we nearly always fall back on guessing. “Guessing” is usually called “decision making” or “expert opinion.”

But this does not imply, and it is not true, that probability is subjective. There is also the, potentially very large, problem that the elicitation makes you too certain because of the quest for quantification (the point of the original post).

This following is an excerpt from my forthcoming introductory book; this comes after discussions of frequentism and logical probability.

**Why probability can’t be subjective**

If 3 out of 4 dentists agree that using Dr Johnston’s Whitening Powder makes for shiny teeth, what is the probability that *your* dentist thinks so? Given *only* the evidence (premises) that 3 out of 4 etc., then we know the probability is 0.75 that your dentist likes Dr Johnston’s Whitening Powder.

But what if you learned your dentist had just attended an “informational seminar” (with free lunch) sponsored by Galaxy Pharmaceuticals, the manufacturer of Dr Johnston’s Whitening Powder? This introduces new evidence, and will therefore modify the probability that your doctor would recommend Dr Johnston’s.

It may suddenly seem that probability *is* a matter of belief, of subjective feeling, because different people will have different opinions on how the free lunch will effect the doctor’s endorsement. Probability cannot be a matter of free choice, however. For example, knowing only that a die has 6 sides, and knowing *nothing else* except that the outcome of the die toss is contingent, then the probability of seeing a 6 is 1 in 6, or about 0.17, regardless of what you or I or anybody thinks. [This is from a discussion of logical probability where the evidence is “A die which will be tossed once has six sides, just one of which is labeled ‘6’” and we want the probability of “We see a 6”, which, given the explicit evidence, is 1/6.]

After you learn of your doc’s cozying up to the pharmaceutical representative, *you* would be inclined to increase your probability that he would tout Dr Johnston’s to, say, the extent of 0.95. *I* may come to a different conclusion, say, 0.76 (just slightly higher). Why? Because we are using *different sets or collections of information*, different evidence or premises, which naturally change our probability assessments. You might know more about pharmaceutical companies than I do, for example, and this causes you to be more cynical, whereas I know more about the purity and selflessness of doctors, and this causes me to be trusting.

But, if I agreed with you about the new evidence, and I felt it was as relevant as you did, then we would share the same probability that the conclusion was true. This, of course, is very unlikely to happen. Rarely will two people agree on a list of premises when the argument involves human affairs, and so it is natural that for most complex things, people will come to different probabilities that the conclusions are true. Does this remind you of politics?

Because people never agree on the set of premises—and they cannot loosely agree on them, they have to agree on them *exactly*—probabilities will differ. In this sense, probabilities are subjective—rather, it is the choice of premises that is subjective. The probabilities assigned to a conclusion *given* a set of premises is not. The probability of a conclusion always follows logically from the given premises.