I'm a biochemist and I usually compare the variability of my measurements in terms of coefficient of variation (CV) since I can "visualize" the deviations more easily in terms of percentages deviation from my mean value. Now, I measured an analyte 20 times in a sample in one month, and again 20 times the second month. Now I'd like to calculate the mean CV of those two sets of measurements. I know I can get a pooled sd and then maybe get the arithmetic mean of two means to calculate at the end mean or pooled CV, but I suppose I cannot do that, true? Can I just get mean CV (add, and divide by two?). Statisticians are usually in terms of squares, adding and square-rooting sd-s or variances, am I right? And for what is worth, I suppose also that if I combine all data in one set, I'd probably get different sd and CV?