High Quality Content by WIKIPEDIA articles! Calculation of the sum of normally distributed random variables is the simplest instance of random variable arithmetic, which can be quite complex based on the distributions of the random variables involved and their relationships. This means that the sum of two independent normally distributed random variables is normal, with mean the sum of the two means, and the variance the sum of the two variances (i.e., the square of the standard deviation is the sum of the squares of the standard deviations).