Willmott et al. [Willmott CJ, Robeson SM, Matsuura K. 2012. A refined index of model performance. International Journal of Climatology, forthcoming. DOI:10.1002/joc.2419.] recently suggest a refined index of model performance (d_r) that they purport to be superior to other methods. Their refined index ranges from − 1.0 to 1.0 to resemble a correlation coefficient, but it is merely a linear rescaling of our modified coefficient of efficiency (E₁) over the positive portion of the domain of d_r. We disagree with Willmott et al. (2012) that d_r provides a better interpretation; rather, E₁ is more easily interpreted such that a value of E₁ = 1.0 indicates a perfect model (no errors) while E₁ = 0.0 indicates a model that is no better than the baseline comparison (usually the observed mean). Negative values of E₁ (and, for that matter, d_r < 0.5) indicate a substantially flawed model as they simply describe a ‘level of inefficacy’ for a model that is worse than the comparison baseline. Moreover, while d_r is piecewise continuous, it is not continuous through the second and higher derivatives. We explain why the coefficient of efficiency (E or E₂) and its modified form (E₁) are superior and preferable to many other statistics, including d_r, because of intuitive interpretability and because these indices have a fundamental meaning at zero.

We also expand on the discussion begun by Garrick et al. [Garrick M, Cunnane C, Nash JE. 1978. A criterion of efficiency for rainfall-runoff models. Journal of Hydrology 36: 375-381.] and continued by Legates and McCabe [Legates DR, McCabe GJ. 1999. Evaluating the use of “goodness-of-fit” measures in hydrologic and hydroclimatic model validation. Water Resources Research 35(1): 233-241.] and Schaefli and Gupta [Schaefli B, Gupta HV. 2007. Do Nash values have value? Hydrological Processes 21: 2075-2080. DOI: 10.1002/hyp.6825.]. This important discussion focuses on the appropriate baseline comparison to use, and why the observed mean often may be an inadequate choice for model evaluation and development.