Standard Deviation of Subtraction


Thanks for looking at my post. I am analyzing data for filtration through a set of membranes of different sizes. To get the range of filtering efficiency (for example what passes between 1 and 5 mm and what passes between 5 and 10 mm) I need to subtract the values.

Each filtration was done in triplicates and I have the mean and standard deviation of each filtration run.

My question is how do I calculate the SD when I subtract the means. What is the SD then? Which formula to use, etc. Thank you,



TS Contributor
Whenever two normal variates are added or subtracted, the difference distribution or summed distribution will have a SD that is equal to the square root of the sum of the variances of the two original variates:

SD = sqrt(var1 + var2)

where var = SD^2