Spearman's correlation (Hogg and Craig, 1994) and Kendall's tau (Kendall, 1938) have been widely used in many regression problems in NLP (Albrecht and Hwa, 2007;Yogatama et al., 2011;, and here we use them to measure the quality of predicted valuesŷ by comparing to the vector of ground truth y.
to Pearson's correlation, Spearman's correlation has no assumptions on the relationship of the two measured variables.