- Probability
- The likelihood that something will happen. For example, a probability of less than .05 indicates that the probability of something occurring by chance alone is less than 5 in 100, or 5 percent. This level of probability is usually taken as the level of biologic significance, so a higher incidence may be considered meaningful. The abbreviation for probability is P.
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**1.**A*measure*, ranging from 0 to 1, of the*likelihood*of truth of a*hypothesis*or statement.**2.**The*limit*of the relative*frequency*of an*event*in a*sequence*of N*random*trials as N approaches*infinity*.-**conditional p.**a p. quoted when the*range*of choices admitted is restricted, i.e., conditional;*thus*, the p. of the child of a color-blind man inheriting the*gene*is 1/2 if the child is*female*and almost 0 if the child is*male*.-the p. that two or more outcomes are realized jointly.*joint*p.-a p. of an outcome based either on unassailable*objective*p.*theory*or extensive*empiric**experience*of exactly the same*combination*of circumstances; the notion also implies that the realization concerned has not been effected and therefore even in*principle*not known with certainty.-**personal p.**an*idiosyncratic*judgment about the outcome of an*event*; it may include evidence too subtle to be disposed of in a*subjective*p..-the best*posterior*p.*rational**assessment*of the p. of an outcome on the*basis*of established knowledge modified and brought up to date. SEE ALSO: prior p.,*Bayes*theorem. Cf.:Bayes theorem.-**prior p.**the best*rational**assessment*of the p. of an outcome on the*basis*of established knowledge before the*present**experiment*is performed. For instance, the prior p. of the*daughter*of a*carrier*of*hemophilia*being herself a*carrier*of*hemophilia*is 1/2. But if the*daughter*already has an affected son, the*posterior*p. that she is a*carrier*is unity, whereas if she has a*normal*child, the*posterior*p. that she is a*carrier*is 1/3. See*Bayes*theorem.-a fair statement of the*subjective*p.*odds*that a*rational*, well-informed person would give or*take*for the outcome of an*experiment*. The*experiment*may be unique and not rationally understood (precluding both theoretically*sound*predication and*empirical*experience). The formulation is applicable to experiments that have been carried out but the outcome unknown. (For instance, a certain statement about the sex of the*fetus*early in*pregnancy*is established but perhaps not accessible until*amniocentesis*can be done.) Unlike personal probably, the*subjective*p. should be the same from all competent counselors in possession of the same evidence.* * *

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prob·a·bil·i·ty (prob″ə-bilґĭ-te) [L.*probabilis*probable, from*probare*to test or examine] the likelihood of occurrence of a specified event; it is often represented as a number between 0 (never) and 1 (always) that corresponds to the long-run trial at which an event occurs in a sequence of random independent trials under identical conditions, as the number of trials approaches infinity. Symbol*P.*

*Medical dictionary.
2011.*

**Synonyms**: