 For example, if the query is about copper mines and the top several documents are all about mines in Chile, then there may be query drift in the direction of documents on Chile.In addition, if the words added to the original query are unrelated to the query topic, the quality of the retrieval is likely to be degraded, especially in Web search, where web documents often cover multiple different topics.To improve the quality of expansion words in pseudo-relevance feedback, a positional relevance feedback for pseudo-relevance feedback has been proposed to select from feedback documents those words that are focused on the query topic based on positions of words in feedback documents.