# Calculation of the standard deviation. What am I doing wrong?

I have the following calculation example for the standard deviation, but somehow my result differs from the result in the Excel calculation.

The example contains sample values for a portfolio calculation.

Given are portfolio daily performances:

• Tuesday: 0.00%
• Wednesday: 0.722 %
• Thursday: -0.791 %

Step 1: Calculating the mean value
Mean = $\frac{0 + 0.722 + (-0.791)}{3} = -\frac{0.069}{3} = -0.023$%

-> here I have the same value as Excel

Step 2: Calculating the standard deviation $\displaystyle \sqrt {\frac {(0-(-0.023))^2+(0.722-(-0.023))^2+(-0.791-(-0.023))^2} {3-1}} = \sqrt {\frac {0.023^2 + 0.745^2+ (-0.768)^2} {2}} = \sqrt {\frac {0.0005 + 0.555 + 0.5898} {2}} = \sqrt {\frac {1.1453} {2}} =\sqrt {0.57265} \approx 0.7567$

In this step using the function STABW.S Excel gets a result 0.0076. What am I doing wrong?

Thanks

• 1% = 0.01 [padding so that the comment is long enough] – D. Thomine Oct 19 '16 at 8:50
• You forgot that % is a division by $100$, so $(0.23\%)^2 = 0.0023^2 = 0.00000529$ and not $=(0.23)^2$ – gammatester Oct 19 '16 at 8:51