Relevance Feedback and Query Expansion Recap: A combined measure: F Combined measure that assesses precision/recall tradeoff is F measure weighted harmonic mean F (B2+1)PR a+(1-a) BP+R R People usually use balanced F, measure e,withβ=1orω= Harmonic mean is a conservative averageRelevance Feedback and Query Expansion 4 Recap: A combined measure: F ▪ Combined measure that assesses precision/recall tradeoff is F measure (weighted harmonic mean): ▪ People usually use balanced F1 measure ▪ i.e., with  = 1 or  = ½ ▪ Harmonic mean is a conservative average P R PR P R F + + = + − = 2 2 ( 1) 1 (1 ) 1 1    