I noticed that in 3dMean the help file states that the option -sd (or -stdev) calculates "the standard deviation (variance/n-1)." It is my understanding that in order to get the standard deviation you simply take the square root of the variance, rather than what the help file states, which is divide the variance by n-1. I used 3dcalc to go through the individual steps to calculate the variance and standard deviation in my sample, and it seemed that 3dMean is indeed dividing the variance by n-1 rather than taking the square root in order to get the standard deviation. Is there a specific reason why this would be the case?

Thank you,
Kara