Probability in Horse Racing

A philosopher’s guide to betting on the KY Derby

Last year for the April Derby Issue, I wrote a column on probability, but that was scientific, or logical, probability. This time, I decided to write about something far more important: probability in horse racing. In that last column, I distinguished two kinds of probability, Classical and Relative Frequency. Briefly, classical probability applies to cases where there are a finite number of possible outcomes, each equally likely, like flipping a coin or rolling a die. Relative frequencies are those cases in which there are many trials, and we count how often a particular outcome occurs.

Clearly, neither of these work in the case of horse racing. If we used the classical approach, each horse in a race would be assigned the same odds, but we do not believe that every horse has the same chance of winning. And we cannot run the same race many times over to determine how often Olivia wins.

So, what do the odds in a horse race mean? This kind of probability is called “Subjective” and reflects the beliefs of the people setting the odds…