# If you add two independent random variables, what is the

If you add two independent random variables, what is the standard deviation of the combined distribution, if the standard deviations of the two original distributions were, for example, 7 and 5?
You can still ask an expert for help

• Questions are typically answered in as fast as 30 minutes

Solve your problem for the price of one coffee

• Math expert for every subject
• Pay only if we can solve it

Georgia Ingram

You cannot just add the standard deviations.
Instead, you add the variances. Those are built up from the squared differences between every individual value from the mean (the squaring is done to get positive values only, and for other reasons, that I won't delve into).
Standard deviation is defined as the square root of the variance .
The other way around, variance is the square of SD.
So:
- You square the individual SD's to get the variances
- Then you add these together to get the total variance
- Then you take the square root to get the total SD
Total variance $={7}^{2}+{5}^{2}=49+25=74$
Total SD $=\sqrt[2]{74}=8.60$