Statistics for Maintenance Modelling: Basic Probability Concepts

# The Probability Mass Function¤

For a discrete random variable X, the probability mass function is defined as:

P(x) = P{X = x}

This function says that the probability mass function for x is equal to the probability that the value of the observed random variable X equals the value x. The probability mass function is positive or zero for all x. Since X must take on one of the values xi, we have:

The chart on the right shows the probability mass function for the sum of two dice. The discrete random variable (X) is the sum of the dice, with possible values of x ranging from 2 to 12. P(xi) is calculated by summing the probabilities of the different roles of the die that result in xi.