The ratio developed by dividing the difference between the number of documents retrieved and the number of relevant documents retrieved by the first number, i.e., a measure of the efficiency of the information retrieval system. A zero would be optimum.

• 量度信息检索系统效率的指数，是检得文件数与检得相关文件数之差除以检得文件数所得之值，此指数为零时为效率最佳。