Dispersion (variance)

$DX = (x_1^2 * p_1 + x_2^2 * p_2 + x_3^2 * p_3) - (EX)^2$

DX - dispersion
EX - mathematical expectation
x1, x2, x3 ... - posible values of event
p1, p2, p3 ... - probabilities of event