【正确答案】一般的说，当所要测定的特征能很精确界定时，测试最为有效；而当所要测定或预测的东西不能明确地界定时，测试的效果则最差。

【答案解析】[精解] In general，the tests work most effectively when the qualities to be measured can be most precisely defined and least effectively when what is to be measured or predicted can not be well defined．
句子分析：
一、句子可以拆分为四段：In general，the tests work most effectively／when the qualities to be measured can be most precisely defined／and least effectively／when what is to be measured or predicted can not be well defined．
二、句子的结构：
1)主干结构是一个排比句the tests work most effectively when．．．and least effectively when．．．
2)连词when 引导的是状语从句，意思是当……的时候，从句中都是被动结构。
3)what is to be measured or predicted 是第二个从句的主语。
三、词的处理：
In general一般来说；
work most effectively 最为有效；
be most precisely defined 很精确的界定；
measured 测定；