There is a theorem that says: "Let X and Y be random variables. If X ≤ Y, then E(X) ≤ E(Y)." But I don't really understand the meaning of "X ≤ Y". What does it mean? For example, if X takes on the values 0,1,2,3, and Y takes on the values -1,2,5. Is X ≤ Y?? Any help is appreciated!