[Probability] Basic Concept - Marginal Probability

Summation over other random variables. For discrete random variables:

Wikipedia: Marginal Distribution
The marginal distribution of a subset of a collection of random variables is the probability distribution of the variables contained in the subset.
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Reference: 高應大李明聰老師
兩個或兩個以上的樣本空間中，僅考慮某一個條件下個別發生的機率。