Relevance Feedback and Query Expansion Recap: Unranked retrieval evaluation Precision and recall Precision fraction of retrieved docs that are relevant P(relevant retrieved Recall fraction of relevant docs that are retrieved P(retrieved relevant Relevant Nonrelevant Retrieved Not Retrieved fn Precision P= tp/tp fp) Recall r=tp/tp+ fn)Relevance Feedback and Query Expansion 3 Recap: Unranked retrieval evaluation: Precision and Recall ▪ Precision: fraction of retrieved docs that are relevant = P(relevant|retrieved) ▪ Recall: fraction of relevant docs that are retrieved = P(retrieved|relevant) ▪ Precision P = tp/(tp + fp) ▪ Recall R = tp/(tp + fn) Relevant Nonrelevant Retrieved tp fp Not Retrieved fn tn