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Sum of normally distributed random variables
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Sum of normally distributed random variables : ウィキペディア英語版
Sum of normally distributed random variables

In probability theory, calculation of the sum of normally distributed random variables is an instance of the arithmetic of random variables, which can be quite complex based on the probability distributions of the random variables involved and their relationships.
==Independent random variables==
If ''X'' and ''Y'' are independent random variables that are normally distributed (and therefore also jointly so), then their sum is also normally distributed. i.e., if
:X \sim N(\mu_X, \sigma_X^2)
:Y \sim N(\mu_Y, \sigma_Y^2)
:Z=X+Y,
then
:Z \sim N(\mu_X + \mu_Y, \sigma_X^2 + \sigma_Y^2).
This means that the sum of two independent normally distributed random variables is normal, with its mean being the sum of the two means, and its variance being the sum of the two variances (i.e., the square of the standard deviation is the sum of the squares of the standard deviations).
Note that the result that the sum is normally distributed requires the assumption of independence, not just uncorrelatedness; two separately (not jointly) normally distributed random variables can be uncorrelated without being independent, in which case their sum can be non-normally distributed (see Normally distributed and uncorrelated does not imply independent#A symmetric example). The result about the mean holds in all cases, while the result for the variance requires uncorrelatedness, but not independence.

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