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I have been using the Root Mean Squared Error (RMSE) to measure the accuracy of values predicted using a model. I understand that the value returned is using the units of my measures (rather than a percentage). However, I would like to quote my values as a percentage. The approach that I have taken is to normalize the RMSE by the mean value of my observations.

Is there a term for RMSE/mean ?

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Yes, it is called the coefficient of variation. See this question for some discussion about this parameter, or read the Wikipedia entry.

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    $\begingroup$ +1. Isn't also called: relative root mean square error (rRMSE)? , cc/ @celenius. $\endgroup$ – Andre Silva Jan 30 '14 at 11:28
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It is called Normalised Root Mean Square Error (nRMSE) or Relative Root Mean Square Error (rRMSE). You can use either mean or range as the denominator. Read the Wikipedia article

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In my field (analytical chemistry), absolute error / absolute value = relative error, so relative RMSE [at mean x] would be understood easily.

I'd clarify that the value I divide by is the average, as often the relative error at the extreme values is used:

  • error specification of measuring instruments often is relative error at maximum value
  • in (chemical-analytical) calibration the relative error at the limit of quantitation or the lower limit of the actual calibration is important.
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  • $\begingroup$ I'm not sure if there is a standard term in my field, so I will probably use relative error as it sounds a little more self-explanatory. $\endgroup$ – celenius Apr 21 '12 at 13:37

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