How do I calculate the coefficient of variation, CV (or Relative Standard Deviation, RSD) for a subset? e.g.
group = A A A B B B X = 1, 2, 3, 4, 5, 6
Do I use the mean of the subset
CVA = sdA / meanA = 1 / 2 = 50%
...or the grand mean of the data
CVA = sdA / meanX = 1 / 3.5 = 29%
I found only one paper suggesting it is the grand mean here, but most sources simply don't mention subsets...