常用的長期趨勢測定的方法有哪些？各有什么特點？

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 cumulative frequency of the third class？（）

A cumulative frequency polygon is also called （）.

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 （）.

If data are grouped into intervals and the number of items in each group is listed, this could be called a （）

某企業(yè)的生產(chǎn)計劃完成百分比采用如下分組，請指出哪項是正確的（）

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

A summary measure that is computed from a sample to describe a characteristic of the population is called（）

One advantage of a stem and leaf plot over a frequency distribution is that （）.

為什么在計算平均指標(biāo)的基礎(chǔ)上還有必要進一步計算變異指標(biāo)？