Standard Deviation of Subtraction

#1
Hello,

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,

--Alex
 

JohnM

TS Contributor
#2
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