# Probability

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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.
- joint p. the p. that two or more outcomes are realized jointly.
- objective p. a p. of an outcome based either on unassailable 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..
- posterior p. the best 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.
- subjective p. a fair statement of the 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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n.

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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.

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