为了获得有实用价值的灵敏度及特异度，在筛查阴性者中抽取一定比例样本进行诊断试验是必要的。

In order to obtain the sensitivity and the specificity, which are useful for practice, it is necessary that a subsample should be selected in the screened negative subjects for diagnoses.

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科学家们从乌鸦那里收集了样本，以估计它们食物中幼虫的比例。

Scientists have collected samples from crows in order to estimate the proportion of larvae in their diet.

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查看一下统计样本数量，参考班级中已经就业的毕业生比例，以及他们的工作类型。

Check the sample size, look at the percentage of the class with jobs, and look at what jobs they're getting.

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