Obsolete term for the probability distribution of an unobserved variable

Inverse probability

Inverse probability

In <a href="/en/Probability_theory" title="Probability theory" class="wl">probability theory</a>, inverse probability is an obsolete term for the <a href="/en/Probability_distribution" title="Probability distribution" class="wl">probability distribution</a> of an unobserved variable.

<div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none" about="#mwt1">Obsolete term for the probability distribution of an unobserved variable</div>
Today, the problem of determining an unobserved variable (by whatever method) is called <a href="/en/Inferential_statistics" title="Inferential statistics" class="mw-redirect wl">inferential statistics</a>, the method of inverse probability (assigning a probability distribution to an unobserved variable) is called <a href="/en/Bayesian_probability" title="Bayesian probability" class="wl">Bayesian probability</a>, the "distribution" of data given the unobserved variable is rather the <a href="/en/Likelihood_function" title="Likelihood function" class="wl">likelihood function</a> (which is not a probability distribution), and the distribution of an unobserved variable, given both data and a <a href="/en/Prior_distribution" title="Prior distribution" class="mw-redirect wl">prior distribution</a>, is the <a href="/en/Posterior_distribution" title="Posterior distribution" class="mw-redirect wl">posterior distribution</a>. The development of the field and terminology from "inverse probability" to "Bayesian probability" is described by <a href="#CITEREFFienberg2006" target="_blank">Fienberg (2006)</a>.
<figure class="mw-halign-right wiki-thumb"><a href="/en/File:Youngronaldfisher2.JPG" class="mw-file-description image media-thumb" data-hash="File:Youngronaldfisher2.JPG"><img loading="lazy" src="//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Youngronaldfisher2.JPG/200px-Youngronaldfisher2.JPG" decoding="async" data-file-width="291" data-file-height="408" data-file-type="bitmap" height="280" width="200" srcset="//upload.wikimedia.org/wikipedia/commons/a/aa/Youngronaldfisher2.JPG 1.5x, //upload.wikimedia.org/wikipedia/commons/a/aa/Youngronaldfisher2.JPG 2x" alt="Youngronaldfisher2.JPG"></a><figcaption>Ronald Fisher</figcaption></figure>

The term "inverse probability" appears in an 1837 paper of <a href="/en/Augustus_De_Morgan" title="Augustus De Morgan" class="wl">De Morgan</a>, in reference to <a href="/en/Laplace" title="Laplace" class="mw-redirect wl">Laplace</a>'s method of probability (developed in a 1774 paper, which independently discovered and popularized Bayesian methods, and a 1812 book), though the term "inverse probability" does not occur in these.<a href="#cite_note-FOOTNOTEFienberg20065-1" style="counter-reset: mw-Ref 1;">[1]</a> Fisher uses the term in <a href="#CITEREFFisher1922" target="_blank">Fisher (1922)</a>, referring to "the fundamental paradox of inverse probability" as the source of the confusion between statistical terms that refer to the true value to be estimated, with the actual value arrived at by the estimation method, which is subject to error. Later Jeffreys uses the term in his defense of the methods of Bayes and Laplace, in <a href="#CITEREFJeffreys1939" target="_blank">Jeffreys (1939)</a>. The term "Bayesian", which displaced "inverse probability", was introduced by <a href="/en/Ronald_Fisher" title="Ronald Fisher" class="wl">Ronald Fisher</a> in 1950.<a href="#cite_note-FOOTNOTEFienberg200614-2" style="counter-reset: mw-Ref 2;">[2]</a> Inverse probability, variously interpreted, was the dominant approach to statistics until the development of <a href="/en/Frequentist_probability" title="Frequentist probability" class="wl">frequentism</a> in the early 20th century by <a href="/en/Ronald_Fisher" title="Ronald Fisher" class="wl">Ronald Fisher</a>, <a href="/en/Jerzy_Neyman" title="Jerzy Neyman" class="wl">Jerzy Neyman</a> and <a href="/en/Egon_Pearson" title="Egon Pearson" class="wl">Egon Pearson</a>.<a href="#cite_note-FOOTNOTEFienberg20064.1_Frequentist_Alternatives_to_Inverse_Probability,_pp._7–9-3" style="counter-reset: mw-Ref 3;">[3]</a> Following the development of frequentism, the terms <a href="/en/Frequentist_inference" title="Frequentist inference" class="wl">frequentist</a> and <a href="/en/Bayesian_statistics" title="Bayesian statistics" class="wl">Bayesian</a> developed to contrast these approaches, and became common in the 1950s.

In probability theory, inverse probability is an obsolete term for the probability distribution of an unobserved variable. Today, the problem of determining an unobserved variable is called inferential statistics, the method of inverse probability is called Bayesian probability, the "distribution" of data given the unobserved variable is rather the likelihood function , and the distribution of an unobserved variable, given both data and a prior distribution, is the posterior distribution. The development of the field and terminology from "inverse probability" to "Bayesian probability" is described by Fienberg . The term "inverse probability" appears in an 1837 paper of De Morgan, in reference to Laplace's method of probability , though the term "inverse probability" does not occur in these. Following the development of frequentism, the terms frequentist and Bayesian developed to contrast these approaches, and became common in the 1950s. 