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How to calculate the standard deviation of stock returns?
For implied volatility it is okey to use Black and scholes but what to do with the historical volatility which carry the effect of past prices as a predictor of future prices.And then precisely the conditional historical volatility.i suggest that you must go with the process like, for stock returns  1) first download stock prices into excel sheet 2) take the natural log of (P1/po) 3) calculate average of the sample  4) calculate square of (X-Xbar) 5) take square root of this and you will get the standard deviation of your required data.