8. The average age of chief executive officers (CEO’s) in a large sample of companies is 57. The average age of CEO’s in those same companies 20 years ago was approximately eight years younger. On the basis of those data, it can be concluded that CEO’s in general tend to be older now.
Which of the following casts the most doubt on the conclusion drawn above?
(A) The dates when the CEO’s assumed their current positions have not been specified.
(B) No information is given concerning the average number of years that CEO’s remain in office.
(C) The information is based only on companies that have been operating for at least 20 years.
(D) Only approximate information is given concerning the average age of the CEO’s 20 years ago.（C）
(E) Information concerning the exact number of companies in the sample has not been given.
为什么a不对呢？我觉得没有给出ceo就职的年龄也能说明
结论不对阿，比如20年前的资料是那些人才当上ceo而20年后这些已经当了10年了
。。。。c我倒是理解，是样本特殊的问题
前提：样本中公司的CEO年纪比20年前大了 结论：CEO年纪普遍增大了
削弱结论，就应说明样本不具普遍性。C:样本是运营20年以上的公司。老公司不能代表所有公司－》削弱结论
ABDE犯了同一个错误。以E为例Information concerning the exact number of companies in the sample has not been given.如果这个样本数目很小，当然是削弱结论；但如果这样本数目很大，就加强结论了。你不能凭E那一句话判断它是削弱了还是加强了结论。
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