Combine standard deviations

Hello,

I'm trying to replicate the formulas from here in order to calculate the weighted standard deviation from different samples. The formula is as follows:

image026.gif

I've put together an example workbook and isolated the problem that the error is caused by trying to get the square root of a negative number. The culprit seems to be:

Formula:

- (3*A2*D2*G2)

Can anyone help me to correct this formula?

Thanks.

abousetta