Conditional event algebra

Conditional event algebra

A conditional event algebra (CEA) is an algebraic structure whose domain consists of logical objects described by statements of forms such as "If A, then B," "B, given A," and "B, in case A." Unlike the standard Boolean algebra of events, a CEA allows the defining of a probability function, P, which satisfies the equation P(If A then B) = P(A and B) / P(A) over a usefully broad range of conditions.


Standard probability theory

In standard probability theory, one begins with a set, Ω, of outcomes (or, in alternate terminology, a set of possible worlds) and a set, F, of some (not necessarily all) subsets of Ω, such that F is closed under the countably infinite versions of the operations of basic set theory: union (∪), intersection (∩), and complementation ( ′). A member of F is called an event (or, alternatively, a proposition), and F, the set of events, is the domain of the algebra. Ω is, necessarily, a member of F, namely the trivial event "Some outcome occurs."

A probability function P assigns to each member of F a real number, in such a way as to satisfy the following axioms:

For any event E, P(E) ≥ 0.
P(Ω) = 1
For any countable sequence E1, E2, ... of pairwise disjoint events, P(E1E2 ∪ ...) = P(E1) + P(E2) + ....

It follows that P(E) is always less than or equal to 1. The probability function is the basis for statements like P(AB′) = 0.73, which means, "The probability that A but not B is 73%."

Conditional probabilities and probabilities of conditionals

The statement "The probability that if A, then B, is 24%" means (put intuitively) that event B occurs in 24% of the outcomes where event A occurs. The standard formal expression of this is P(B|A) = 0.24, where the conditional probability P(B|A) equals, by definition, P(AB) / P(A).

It is tempting to write, instead, P(AB) = 0.24, where AB is the conditional event "If A, then B." That is, given events A and B, one might posit an event, AB, such that P(AB) could be counted on to equal P(B|A). One benefit of being able to refer to conditional events would be the opportunity to nest conditional event descriptions within larger constructions. Then, for instance, one could write P(A ∪ (BC)) = 0.51, meaning, "The probability that either A, or else if B, then C, is 51%."

Unfortunately, philosopher David Lewis showed that in orthodox probability theory, only certain trivial Boolean algebras with very few elements contain, for any given A and B, an event X which satisfies P(X) = P(B|A). Later extended by others, this result stands as a major obstacle to any talk about logical objects that can be the bearers of conditional probabilities.

The construction of conditional event algebras

The classification of an algebra makes no reference to the nature of the objects in the domain, being entirely a matter of the formal behavior of the operations on the domain. However, investigation of the properties of an algebra often proceeds by assuming the objects to have a particular character. Thus, the canonical Boolean algebra is, as described above, an algebra of subsets of a universe set. What Lewis in effect showed is what can and cannot be done with an algebra whose members behave like members of such a set of subsets.

Conditional event algebras circumvent the obstacle identified by Lewis by using a nonstandard domain of objects. Instead of being members of a set F of subsets of some universe set Ω, the canonical objects are normally higher-level constructions of members of F. The most natural construction, and historically the first, uses ordered pairs of members of F. Other constructions use sets of members of F or infinite sequences of members of F.

Specific types of CEA include the following (listed in order of discovery):

Shay algebras
Calabrese algebras
Goodman-Nguyen-van Fraassen algebras
Goodman-Nguyen-Walker algebras

CEAs differ in their formal properties, so that they cannot be considered a single, axiomatically characterized class of algebra. Goodman-Nguyen-van Frassen algebras, for example, are Boolean while Calabrese algebras are non-distributive. The latter, however, support the intuitively appealing identity A → (BC) = (AB) → C, while the former do not.


Goodman, I. R., R. P. S. Mahler, and H. T. Nguyen. 1999. "What is conditional event algebra and why should you care?" SPIE Proceedings, Vol 3720.

Lewis, David K. 1976. "Probabilities of conditionals and conditional probabilities." Philosophical Review 85: 297-315.

Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать реферат

Look at other dictionaries:

  • Goodman-Nguyen-van Fraassen algebra — A Goodman Nguyen van Fraassen algebra is a type of conditional event algebra (CEA) that embeds the standard Boolean algebra of unconditional events in a larger algebra which is itself Boolean. The goal (as with all CEAs) is to equate the… …   Wikipedia

  • Boolean algebra (structure) — For an introduction to the subject, see Boolean algebra#Boolean algebras. For the elementary syntax and axiomatics of the subject, see Boolean algebra (logic). For an alternative presentation, see Boolean algebras canonically defined. In abstract …   Wikipedia

  • Conditional probability — The actual probability of an event A may in many circumstances differ from its original probability, because new information is available, in particular the information that an other event B has occurred. Intuition prescribes that the still… …   Wikipedia

  • Conditional expectation — In probability theory, a conditional expectation (also known as conditional expected value or conditional mean) is the expected value of a real random variable with respect to a conditional probability distribution. The concept of conditional… …   Wikipedia

  • Conditional independence — These are two examples illustrating conditional independence. Each cell represents a possible outcome. The events R, B and Y are represented by the areas shaded red, blue and yellow respectively. And the probabilities of these events are shaded… …   Wikipedia

  • Event (probability theory) — In probability theory, an event is a set of outcomes (a subset of the sample space) to which a probability is assigned. Typically, when the sample space is finite, any subset of the sample space is an event ( i . e . all elements of the power set …   Wikipedia

  • List of mathematics articles (C) — NOTOC C C closed subgroup C minimal theory C normal subgroup C number C semiring C space C symmetry C* algebra C0 semigroup CA group Cabal (set theory) Cabibbo Kobayashi Maskawa matrix Cabinet projection Cable knot Cabri Geometry Cabtaxi number… …   Wikipedia

  • Probability space — This article is about mathematical term. For the novel, see Probability Space (novel). In probability theory, a probability space or a probability triple is a mathematical construct that models a real world process (or experiment ) consisting of… …   Wikipedia

  • Independence (probability theory) — In probability theory, to say that two events are independent intuitively means that the occurrence of one event makes it neither more nor less probable that the other occurs. For example: The event of getting a 6 the first time a die is rolled… …   Wikipedia

  • Bayes' theorem — In probability theory, Bayes theorem (often called Bayes law after Thomas Bayes) relates the conditional and marginal probabilities of two random events. It is often used to compute posterior probabilities given observations. For example, a… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”