Identify the random variable and its probability distribution
For a discrete distribution, multiply each value by its probability
Add all the products to get the mean
Use the formula ( mu = sum x , P(x) ) for a discrete distribution
For a continuous distribution, multiply each value by the probability density function
Integrate over all possible values to get the mean
Use the formula ( mu = int_{-infty}^{infty} x f(x),dx ) for a continuous distribution
Check that all probabilities or the density function are properly defined over the distribution
Verify the result has the same units as the random variable