The discrete random variables X₁, X₂, ... , X_n are independent if for arbitrary values x₁, x₂, ... , x_n of the variables the probability that X₁ = x₁ and X₂ = x₂, etc., is equal to the product of the probabilities that X_i = x_i for i = 1, 2, ... , n; random variables which are unrelated.