last-7-1-10

meimei5656

A large group of hyperactive children whose regular diets included food containing large amounts of additives was observed by researchers trained to assess the presence or absence of behavior problems. The children were then placed on a low-additive diet for several weeks, after which they were observed again. Originally nearly 60 percent of the children exhibited behavior problems; after the change in diet, only 30 percent did so. On the basis of these data, it can be concluded that food additives can contribute to behavior problems in hyperactive children.


The evidence cited fails to establish the conclusion because


(A) there is no evidence that the reduction in behavior problems was proportionate to the reduction in food-additive intake


(B) there is no way to know what changes would have occurred without the change of diet, since only children who changed to a low-additive diet were studied


(C) exactly how many children exhibited behavior problems after the change in diet cannot be determined, since the size of the group studied is not precisely given


(D) there is no evidence that the behavior of some of the children was unaffected by additives


(E) the evidence is consistent with the claim that some children exhibit more frequent behavior problems after being on the low-additive diet than they had exhibited when first observed


答案B。如何理解请高手指教！谢谢

himba

这里说的其实就是生物学中一个非常浅显的道理
对比试验
举个例子
有人开始吃了个鸡蛋，结果生了孩子
结论，吃了鸡蛋就会生孩子
呵呵，当然不对，再找一个人不吃鸡蛋，她也生了孩子
原文就是这个道理，一定要有一个对比的东西存在在那里，这样才能更说明问题

meimei5656

明白了。谢谢版主！！！

steveyangxt

辛巴班主说得很形象，赞一个。
[em05]
我再从对比关系上分析一下：按照对比规则，只要有一个参照物，那马一个变量A实施以后的情形和实施以前的情形对比即可，即把变量A实施前的情形作为参照物 。
但是，这是在不考虑时间因素的前提下，假如时间也是一个变量B，那变量A实施前的情形就不能作为参照物了。
怎马办呢？那就想办法让时间这个变量不起作用，或者说时间上又可比性。就在时间轴上先取消变量A的作用，让时间变量B单独发生作用，再用它做参照物才行。
-----[U]--------------------------[/U]---------------}时间轴
              A起作用的时间

-----[U]--------------------------[/U]---------------}时间轴
          单让时间变量起作用的时间  
后者做前者的参照。