The basic requirements of patent retrieval are completeness, accuracy, quickness and agility, where completeness means no omission, accuracy means pertinence, quickness means short time, and agility means flexible use of various retrieval elements and strategies. In practice, recall and precision are often used to measure the effect of patent retrieval. Recall is often used to measure the ability of retrieval system and searcher to detect relevant information, and precision is often used to measure the ability of retrieval system and searcher to reject irrelevant information.