Topic summary
Marginal probability
Extracted from the Wikipedia article Marginal distribution.
Marginal probability mass function
Given a known joint distribution of two discreterandom variables, say, X and Y, the marginal distribution of either variable – X for example – is the probability distribution of X when the values of Y are not taken into consideration. This can be calculated by summing the joint probability distribution over all values of Y. Naturally, the converse is also true: the marginal distribution can be obtained for Y by summing over the separate values of X.