# Coefficient of variation for a subset

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...

Many thanks.

• Either could be defensible. Whatever you do, make it clear. – Nick Cox Apr 12 '15 at 21:54