# Bayes’s theorem

### Bayes’s theorem

Bayes’s theorem, in probability theory, a means for revising predictions in light of relevant evidence, also known as conditional probability or inverse probability. The theorem was discovered among the papers of the English Presbyterian minister and mathematician Thomas Bayes and published posthumously in 1763. Related to the theorem is Bayesian inference, or Bayesianism, based on the assignment of some a priori distribution of a parameter under investigation. In 1854 the English logician George Boole criticized the subjective character of such assignments, and Bayesianism declined in favour of “confidence intervals” and “hypothesis tests”—now basic research methods.

As a simple application of Bayes’s theorem, consider the results of a screening test for infection with the human immunodeficiency virus (HIV; see AIDS). Suppose an intravenous drug user undergoes testing where experience has indicated a 25 percent chance that the person has HIV. A quick test for HIV can be conducted, but it is not infallible: almost all individuals who have been infected long enough to produce an immune system response can be detected, but very recent infections may go undetected. In addition, “false positive” test results (that is, a false indication of infection) occur in 0.4 percent of people who are not infected. Hence, positive test results do not prove that the person is infected. Nevertheless, infection seems more likely for those who test positive, and Bayes’s theorem provides a formula for evaluating the probability. The logic of this formula is illustrated in the Bayes’s theorem used for evaluating the accuracy of a medical test [Credit: Encyclopædia Britannica, Inc.]figure and explained as follows.

Suppose that there are 10,000 intravenous drug users in the population, of which 2,500 are infected with HIV. Suppose further that if all 2,500 people are tested, 95 percent (2,375 people) will produce a positive test result. The other 5 percent are known as “false negatives.” In addition, of the remaining 7,500 people who are not infected, about 0.4 percent, or 30 people, will test positive (“false positives”). Since there are 2,405 positive tests in all, the probability that a person testing positive is actually infected can be calculated as 2,375/2,405, or about 98.8 percent.

Applications of Bayes’s theorem used to be limited mostly to such straightforward problems, even though the original version was more complex. There are two key difficulties in extending these sorts of calculations, however. First, the starting probabilities are rarely so easily quantified. They are often highly subjective. To return to the HIV screening described above, a patient might appear to be an intravenous drug user but might be unwilling to admit it. Subjective judgment would then enter into the probability that the person indeed fell into this high-risk category. Hence, the initial probability of HIV infection would in turn depend on subjective judgment. Second, the evidence is not often so simple as a positive or negative test result. If the evidence takes the form of a numerical score, then the sum used in the denominator of the above calculation will have to be replaced by an integral. More complex evidence can easily lead to multiple integrals that, until recently, could not be readily evaluated.

Nevertheless, advanced computing power, along with improved integration algorithms, has overcome most calculation obstacles. In addition, theoreticians have developed rules for delineating starting probabilities that correspond roughly to the beliefs of a “sensible person” with no background knowledge. These can often be used to reduce undesirable subjectivity. These advances have led to a recent surge of applications of Bayes’s theorem, more than two centuries since it was first put forth. It is now applied to such diverse areas as the productivity assessment for a fish population and the study of racial discrimination.

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