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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 cannot be well defined.

昨天句子的解析：

Whether to use tests, other kinds of information, or both in a particular situation depends, therefore, upon the evidence from experience concerning comparative validity and upon such factors as cost and availability.

词汇突破：1. Concerning 关于

          2. comparative validity 相对效度

          3. cost and availability 费用和有无来源

主干确定：Whether to use tests, other kinds of information, or both depends upon the evidence and upon such factors as cost and availability

          结构提取：whether to use A, B, or both depends upon D and E.

          用A用B还是用C取决于D和E；

其他成分：1.from experience concerning comparative validity 定语

          2. in a particular situation状语

参考译文：因此，在某一特定情况下，究竟是采用测试还是其他种类的信息，或是两者同时使用，须凭有关相对效度的经验依据而定，也取决于诸如费用和有无来源等因素。