问答题 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？

问答题 为什么在计算平均指标的基础上还有必要进一步计算变异指标？

单项选择题 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？（）