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读者到图书馆欲借一本书刊,或没发现合适的,或借到书刊,不管是否借到,都算一次需求(按排队论术语,算一次到达).如果书刊恰好已借出,这样虽然没有构成一次流通,但对此书刊也应算作一次需求。这些“失望”的读者数虽不能直接度量,但能在概率意义上度量。如果我们作出合理的假定,即对特定书刊之需求率从统计角度看是常数,不管是否已经借出,这样就能度量“失望”读者数。那种需求率λ相似于排队论中的期望到达率.如果以年为度量单位,则称λ为书刊的年期望需求率.
The reader wants to borrow a book from the library, or finds no suitable one, or borrows it from the books. Whether or not he borrows it, he counts the demand once (in queuing terminology, arrives once). If the book happens to be lent, It does not constitute a circulation, but this book should also be counted as a demand. Although the number of these “disappointed” readers cannot be directly measured, they can be measured in a probabilistic sense. If we make a reasonable assumption that the demand rate for a particular book is statistically constant, whether or not it has been lent, we can measure the number of “disappointed” readers. The demand rate λ is similar to the expected arrival rate in the queuing theory. If the year is the unit of measure, then λ is called the annual expected demand rate of the book.