Portfolio Standard Deviation

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If I have a portfolio of 10 different stocks can I take each individual stock’s standard deviation and average them to find the overall standard deviation of the portfolio?

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Assuming the stock behaviors are independent (in real life they are probably not), the variance of the sum is the sum of the variances. Therefore the standard deviation of the sum is the square root of the sum of the variances.

The other extreme is the situation where all the stock prices are completely (positively) correlated. In this case the deviation of the sum is the sum is the sum of the deviations.