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A capital E with a variable in brackets, meaning the long-run average.
Also known as: Expectation operator, E of X
E[X] is the average value a random variable takes over the long run, weighted by how likely each outcome is. It need not be a value the variable can actually take: the expected value of a fair die is 3.5.
Denotes the expected value of a random variable: the probability-weighted average of its possible values.
Contexts: Mathematics
Need not be an attainable value. A fair six-sided die has expected value 3.5, which no face shows.
A capital letter E followed by a variable name enclosed in square brackets or round brackets.
Both are operators of probability theory applied to the same random variables.