The solid line in Figure 2 shows the recall-at-1220 for a boosted ranker using all monolingual statistics and k bilingual statistics, for increasing k. Bilingual statistics are added according to their individual recall, from best-performing to worst.
the point at k = 0 uses only µ 1 , µ 2 , and µ 3 , the point at k = 1 adds the best individually-performing bilingual statistic (Spanish) as a weak ranker, the next point adds the second-best bilingual statistic (German), etc.