Numerical models of climatic, and hydrologic have grown over the last few decades. There has always been an interest to use Willmott's dimensionless index of agreement in comparing  the models, particularly to determine which models are more accurate.

This can be added easily in XLSTAT.

For more info "Willmott, C. J., Robeson, S. M. and Matsuura, K. (2012), A refined index of model performance. Int. J. Climatol., 32: 2088–2094. doi:10.1002/joc.2419"