One rule that must always be followed in constructing frequency distributions is that （）.

The number of phone calls arriving at a switchboard each hour has been recorded and the following frequency distribution has been developed.What is the approximate range of the number of phone calls arriving each hour？（）

A student achieves an 82 on the first test in a statistics course. From this, she assumes that her average at the end of the semester (after other tests) will be about 82. This is an example of （）.

為什么在計算平均指標的基礎上還有必要進一步計算變異指標？

Which of the following statements is not true？（）

企業(yè)生產一種產品需順次經過四個車間，這四個車間的廢品率分別為1.5%、2.0%、2.0%和1.0%。該企業(yè)生產這種產品的平均廢品率是多少？

對全國各鐵路交通樞紐的貨運量、貨物種類等進行調查，以了解全國鐵路貨運概況。這種調查屬于（）

品合格率指標是（）

If the individual class frequency is divided by the total frequency, the result is the （）.

A cumulative frequency polygon is also called （）.