The variation coefficient, more commonly called the coefficient of variation, is the ratio of the standard deviation to the mean. For a population and a sample, respectively, it is
 |  |  |
(1)
|
 |  |  |
(2)
|
where 
is the population mean and 
is the sample mean. The coefficient
is dimensionless and is unchanged when all observations are multiplied by the same
positive constant, but it is not unchanged by adding a constant. It is undefined
when the relevant mean is zero and can be unstable when that mean is close to zero.
Interpretation also requires care for variables that can have negative means.
