- 1、本文档共47页,可阅读全部内容。
- 2、原创力文档(book118)网站文档一经付费(服务费),不意味着购买了该文档的版权,仅供个人/单位学习、研究之用,不得用于商业用途,未经授权,严禁复制、发行、汇编、翻译或者网络传播等,侵权必究。
- 3、本站所有内容均由合作方或网友上传,本站不对文档的完整性、权威性及其观点立场正确性做任何保证或承诺!文档内容仅供研究参考,付费前请自行鉴别。如您付费,意味着您自己接受本站规则且自行承担风险,本站不退款、不进行额外附加服务;查看《如何避免下载的几个坑》。如果您已付费下载过本站文档,您可以点击 这里二次下载。
- 4、如文档侵犯商业秘密、侵犯著作权、侵犯人身权等,请点击“版权申诉”(推荐),也可以打举报电话:400-050-0827(电话支持时间:9:00-18:30)。
查看更多
湖南大学计算机与通信学院 刘钰峰 回顾 1、中文分词 2、词典压缩 3、posting list压缩 4、tfidf Scoring documents How do we construct an index? What strategies can we use with limited main memory? Scoring We wish to return in order the documents most likely to be useful to the searcher How can we rank order the docs in the corpus with respect to a query? Assign a score – say in [0,1] for each doc on each query Begin with a perfect world – no spammers Nobody stuffing keywords into a doc to make it match queries More on “adversarial IR” under web search Linear zone combinations First generation of scoring methods: use a linear combination of Booleans: E.g., Score = 0.6*sorting in Title + 0.3*sorting in Abstract + 0.05*sorting in Body + 0.05*sorting in Boldface Each expression such as sorting in Title takes on a value in {0,1}. Then the overall score is in [0,1]. Exercise On the query bill OR rights suppose that we retrieve the following docs from the various zone indexes: General idea We are given a weight vector whose components sum up to 1. There is a weight for each zone/field. Given a Boolean query, we assign a score to each doc by adding up the weighted contributions of the zones/fields. Typically – users want to see the K highest-scoring docs. Index support for zone combinations In the simplest version we have a separate inverted index for each zone Variant: have a single index with a separate dictionary entry for each term and zone E.g., Zone combinations index The above scheme is still wasteful: each term is potentially replicated for each zone In a slightly better scheme, we encode the zone in the postings: At query time, accumulate contributions to the total score of a document from the various postings, e.g., Score accumulation As we walk the postings for the query bill OR rights, we accumulate scores for each doc in a linear merge as before. Note: we get both bill and rights in the Title field of doc 3, but score it no higher. Should we give more weight to more hits? Term-document
文档评论(0)