Question: what is a valid discrete probability distribution?
Answer: To be a valid discrete probability distribution, we need: the sum of the probabilities of all the possible values of the random variable to be 1, i.e., X Pr ( X = x ) = 1 ; the probabilities of each possible value of the random variable to lie between 0 and 1, i.e., 0 ≤ Pr ( X = x ) ≤ 1 .