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

企业生产一种产品需顺次经过四个车间，这四个车间的废品率分别为1.5%、2.0%、2.0%和1.0%。该企业生产这种产品的平均废品率是多少？

某班的学习按考试成绩分为60分以下，60-70，70-80，80-90，90-100，这种分组（）

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？（）

Claudia Taylor, Director of Global Industrial Sales in Melbourne, is concerned by a deteriorating sales trend. Specifically, the number of customers is stable at 1500 but they are purchasing less each year. She orders her staff to search for causes of the downward trend by selecting a focus group of 40 industrial customers. For this study, the set of 40 industrial customers is a （）.

要了解某地区全部大学生的就业情况，那么（）

对某班级学生的生活状况进行统计调查，下列标志中属于不变标志的是（）

某企业的生产计划完成百分比采用如下分组，请指出哪项是正确的（）

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

最能反映离散程度的指标是（）