relative standard deviation

I just came to the sad conclusion that I have not been maintaining my statistical skills enough the last years, and therefore I must kindly ask for help in this easy question: How to find standard deviations from variable A for individuals in a group adjusted for effect of variable B? I have a...

This may be a naïve question but I have read in several sources (example) that calculating the relative standard deviation is unreliable if the mean is close to zero. My questions are: 1.) How close should the mean be to zero before RSD(%) becomes unreliable for expressing statistical spread...