Conditional probability

In probability theory, conditional probability is a measure of the probability of an event occurring, given that another event is already known to have occurred. This particular method relies on event A occurring with some sort of relationship with another event B. In this situation, the event A can be analyzed by a conditional probability with respect to B. If the event of interest is and the event is known or assumed to have occurred, "the conditional probability of given ", or "the probability of under the condition ", is usually written as or occasionally. This can also be understood as the fraction of probability B that intersects with A, or the ratio of the probabilities of both events happening to the "given" one happening :
For example, the probability that any given person has a cough on any given day may be only 5%. But if we know or assume that the person is sick, then they are much more likely to be coughing. For example, the conditional probability that someone sick is coughing might be 75%, in which case we would have that = 5% and = 75 %. Although there is a relationship between and in this example, such a relationship or dependence between and is not necessary, nor do they have to occur simultaneously.
may or may not be equal to, i.e., the unconditional probability or absolute probability of. If, then events and are said to be independent: in such a case, knowledge about either event does not alter the likelihood of each other. typically differs from. For example, if a person has dengue fever, the person might have a 90% chance of being tested as positive for the disease. In this case, what is being measured is that if event has occurred, the probability of given that occurred is 90%, simply writing = 90%. Alternatively, if a person is tested as positive for dengue fever, they may have only a 15% chance of actually having this rare disease due to high false positive rates. In this case, the probability of the event given that the event has occurred is 15% or = 15%. It should be apparent now that falsely equating the two probabilities can lead to various errors of reasoning, which is commonly seen through base rate fallacies.
While conditional probabilities can provide extremely useful information, limited information is often supplied or at hand. Therefore, it can be useful to reverse or convert a conditional probability using Bayes' theorem:. Another option is to display conditional probabilities in a conditional probability table to illuminate the relationship between events.

Definition

Conditioning on an event

Kolmogorov">Andrey Kolmogorov">Kolmogorov definition

Given two events and from the sigma-field of a probability space, with the unconditional probability of being greater than zero is the probability of A occurring if B has or is assumed to have happened. A is assumed to be the set of all possible outcomes of an experiment or random trial that has a restricted or reduced sample space. The conditional probability can be found by the quotient of the probability of the joint intersection of events and, that is,, the probability at which A and B occur together, and the probability of :
For a sample space consisting of equal likelihood outcomes, the probability of the event A is understood as the fraction of the number of outcomes in A to the number of all outcomes in the sample space. Then, this equation is understood as the fraction of the set to the set B. Note that the above equation is a definition, not just a theoretical result. We denote the quantity as and call it the "conditional probability of given."

As an axiom of probability

Some authors, such as de Finetti, prefer to introduce conditional probability as an axiom of probability:
This equation for a conditional probability, although mathematically equivalent, may be intuitively easier to understand. It can be interpreted as "the probability of B occurring multiplied by the probability of A occurring, provided that B has occurred, is equal to the probability of the A and B occurrences together, although not necessarily occurring at the same time". Additionally, this may be preferred philosophically; under major probability interpretations, such as the subjective theory, conditional probability is considered a primitive entity. Moreover, this "multiplication rule" can be practically useful in computing the probability of and introduces a symmetry with the summation axiom for Poincaré Formula:

As the probability of a conditional event

Conditional probability can be defined as the probability of a conditional event. The Goodman–Nguyen–Van Fraassen conditional event can be defined as:
It can be shown that
which meets the Kolmogorov definition of conditional probability.

Conditioning on an event of probability zero

If, then according to the definition, is undefined.
The case of greatest interest is that of a random variable, conditioned on a continuous random variable resulting in a particular outcome. The event has probability zero and, as such, cannot be conditioned on.
Instead of conditioning on being exactly, we could condition on it being closer than distance away from. The event will generally have nonzero probability and hence, can be conditioned on.
We can then take the limit
For example, if two continuous random variables and have a joint density, then by L'Hôpital's rule and Leibniz integral rule, upon differentiation with respect to :
The resulting limit is the conditional probability distribution of given and exists when the denominator, the probability density, is strictly positive.
It is tempting to define the undefined probability using limit, but this cannot be done in a consistent manner. In particular, it is possible to find random variables and and values, such that the events and are identical but the resulting limits are not:
The Borel–Kolmogorov paradox demonstrates this with a geometrical argument.

Conditioning on a discrete random variable

Let be a discrete random variable and its possible outcomes denoted. For example, if represents the value of a rolled dice then is the set. Let us assume for the sake of presentation that is a discrete random variable, so that each value in has a nonzero probability.
For a value in and an event, the conditional probability
is given by.
Writing
for short, we see that it is a function of two variables, and.
For a fixed, we can form the random variable. It represents an outcome of whenever a value of is observed.
The conditional probability of given can thus be treated as a random variable with outcomes in the interval. From the law of total probability, its expected value is equal to the unconditional probability of.

Partial conditional probability

The partial conditional probability
is about the probability of event
given that each of the condition events
has occurred to a degree
that might be different from 100%. Frequentistically, partial conditional probability makes sense, if the conditions are tested in experiment repetitions of appropriate length
. Such
-bounded partial conditional probability can be defined as the conditionally expected average occurrence of event
in testbeds of length
that adhere to all of the probability specifications
, i.e.:
Based on that, partial conditional probability can be defined as
where
Jeffrey conditionalization
is a special case of partial conditional probability, in which the condition events must form a partition:

Example

Suppose that somebody secretly rolls two fair six-sided dice, and we wish to compute the probability that the face-up value of the first one is 2, given the information that their sum is no greater than 5.

Let D₁ be the value rolled on dice 1.
Let D₂ be the value rolled on dice 2.

Probability that D''₁ = 2
Table 1 shows the sample space of 36 combinations of rolled values of the two dice, each of which occurs with probability 1/36, with the numbers displayed in the red and dark gray cells being D₁ + D₂.
D₁ = 2 in exactly 6 of the 36 outcomes; thus P = = :
Probability that D''₁ + D₂ ≤ 5
Table 2 shows that D₁ + D₂ ≤ 5 for exactly 10 of the 36 outcomes, thus P = :
Probability that ''D₁ = 2 given that D''₁ + D₂ ≤ 5
Table 3 shows that for 3 of these 10 outcomes, D₁ = 2.
Thus, the conditional probability P = = 0.3:
Here, in the earlier notation for the definition of conditional probability, the conditioning event B is that D₁ + D₂ ≤ 5, and the event A is D₁ = 2. We have as seen in the table.

Use in inference

In statistical inference, the conditional probability is an update of the probability of an event based on new information. The new information can be incorporated as follows:

Let A, the event of interest, be in the sample space, say.
The occurrence of the event A knowing that event B has or will have occurred, means the occurrence of A as it is restricted to B, i.e..
Without the knowledge of the occurrence of B, the information about the occurrence of A would simply be P
The probability of A knowing that event B has or will have occurred, will be the probability of relative to P, the probability that B has occurred.
This results in whenever P > 0 and 0 otherwise.

This approach results in a probability measure that is consistent with the original probability measure and satisfies all the Kolmogorov axioms. This conditional probability measure also could have resulted by assuming that the relative magnitude of the probability of A with respect to X will be preserved with respect to B.
The wording "evidence" or "information" is generally used in the Bayesian interpretation of probability. The conditioning event is interpreted as evidence for the conditioned event. That is, P is the probability of A before accounting for evidence E, and P is the probability of A after having accounted for evidence E or after having updated P. This is consistent with the frequentist interpretation, which is the first definition given above.