In this paper a new Query Performance Prediction method is introduced. This method is based on the
hypothesis that different score distributions appear for ‘hard’ and ‘easy’ queries. Following we
propose a set of measures which try to capture the differences between both types of distributions,
focusing on the dispersion degree among the scores. We have applied some variants of the classic
standard deviation and have studied methods to find out the most suitable size of the ranking list
for these measures. Finally, we present the results obtained performing the experiments on two
different data-sets.