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If more than one random variable is defined in a random experiment, it is important to distinguish between the joint probability distribution of X and Y and the probability distribution of each variable individually. The individual probability distribution of a random variable is referred to as its marginal probability distribution.
If the conditional distribution of given is a continuous distribution, then its probability density function is known as the conditional density function. [1] The properties of a conditional distribution, such as the moments , are often referred to by corresponding names such as the conditional mean and conditional variance .
That is, the joint distribution is equal to the product of the marginal distributions. [2] Unless it is not clear in context, in practice the modifier "mutual" is usually dropped so that independence means mutual independence. A statement such as " X, Y, Z are independent random variables" means that X, Y, Z are mutually independent.
In probability theory, a probability density function (PDF), density function, or density of an absolutely continuous random variable, is a function whose value at any given sample (or point) in the sample space (the set of possible values taken by the random variable) can be interpreted as providing a relative likelihood that the value of the ...
where (=, =) is the joint probability mass function of X and Y. The sum is taken over all possible outcomes of X . Remark that as above the expression is undefined if P ( Y = y ) = 0 {\displaystyle P(Y=y)=0} .
This measure is also known as the joint probability distribution, the joint distribution, or the multivariate distribution of the random vector. The distributions of each of the component random variables X i {\displaystyle X_{i}} are called marginal distributions .
when the two marginal functions and the copula density function are known, then the joint probability density function between the two random variables can be calculated, or; when the two marginal functions and the joint probability density function between the two random variables are known, then the copula density function can be calculated.
If, on the other hand, we know the characteristic function φ and want to find the corresponding distribution function, then one of the following inversion theorems can be used. Theorem. If the characteristic function φ X of a random variable X is integrable, then F X is absolutely continuous, and therefore X has a probability density function.