  The correlation of the result lists provided by search engines is fundamental
and it has deep and multidisciplinary ramifications. Here, we present automatic
and unsupervised methods to assess whether or not search engines provide
results that are comparable or correlated. We have two main contributions:
First, we provide evidence that for more than 80% of the input queries -
independently of their frequency - the two major search engines share only
three or fewer URLs in their search results, leading to an increasing
divergence. In this scenario (divergence), we show that even the most robust
measures based on comparing lists is useless to apply; that is, the small
contribution by too few common items will infer no confidence. Second, to
overcome this problem, we propose the fist content-based measures - i.e.,
direct comparison of the contents from search results; these measures are based
on the Jaccard ratio and distribution similarity measures (CDF measures). We
show that they are orthogonal to each other (i.e., Jaccard and distribution)
and extend the discriminative power w.r.t. list based measures. Our approach
stems from the real need of comparing search-engine results, it is automatic
from the query selection to the final evaluation and it apply to any
geographical markets, thus designed to scale and to use as first filtering of
query selection (necessary) for supervised methods.
