Commonly adopted metrics for ranking, such as mean average precision (Buckley and Voorhees, 2000) and Normalized Discounted Cumulative Gain (Järvelin and Kekäläinen, 2000), is designed for data sets with human relevance judgment, which is not available to us.
we use the Kendall's tau coefficient (Kendall, 1938;Joachims, 2002) to measure the degree of correlation between two rankings.