The basic requirements of patent retrieval are completeness, accuracy, rapidity and agility. Integrity means no omission, accuracy means pertinence, rapidity 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 ratio is often used to measure the ability of retrieval systems and searchers to detect relevant information, and precision ratio is often used to measure the ability of retrieval systems and searchers to reject irrelevant information.