# What does a negative coefficient of variation mean when calculated from daily returns during a period?

I was wondering what a negative coefficient of variation means when calculated from a set of daily returns for an index?

With coefficient of variation you refer to the relative standard deviation $\frac{\sigma}{\mu}$ I suppose? In this case, negative values occur, as your historical data exhibits a negative drift, which means your estimate of $\mu$ is negative. In my understanding, the coefficient of variation should only be used for data in a ratio scale, or, more general, for data which does not exhibit negative values - this is not really appropriate for return time series.