Normalized Root Mean Square Error (NRMSE) with zero mean of observed value I would like to evaluate the predictive performance of a statistical model using Normalized Root Mean Square Error (NRMSE = RMSE/mean (observed)). However, the mean value of the observation data is all '0' (all observed data are '0'). How can I calculate the NRMSE, otherwise is there any other technique which can represent the relative/percentage value of error with such data aspect?
 A: I think you can, but instead of dividing the RMSE by the mean, you may divide it by (max-min) value
A: I think Euan has a right answer. There are ways to calculate the NRMSE, RMSE/(max()-min()) and RMSE/mean(). You should know which is better to be used in your case. For example, when you are calculating the NRMSE of a house appliance, it is better to use the RMSE/(max()-min()). Because in this way it can show the NRMSE when the appliance is running. The reason why your mean value is 0 could be the data has both positive part and negative part, therefore, I think RMSE/(max()-min()) can show how your data spreads. 
A: Euan Russano suggests dividing by the range of observations which is common (e.g. https://en.wikipedia.org/wiki/Root-mean-square_deviation NRMSD). But this would still be dividing by zero in your case because the range of observations is zero.
other measures of association (like correlation) will also be undefined because the variance is zero.
A: You would normally divide by a measure of "spread". Either max(obs)-min(obs), as already mentioned, or directly the standard deviation of your observations, which is preferred for normally (or quasi-) distributed data. This is objective and gives your NRMSE nice units of "standard deviations of the observed data". You could also divide by the variance. If your observations are not constant, these two quantities should not be zero. Hope it helps.
