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