because people are their own controls here.

因为人们自己就是自己的对照
You're controlling for the factors

你们可以控制这些因素
that might otherwise complicate interpretations

不然它们可能会导致
that you have of associating certain risk factors with disease.

对疾病风险因素的研究分析变得更复杂
Let's just say family history is one predictor of disease.

我们假设家族史也是疾病的一个诱因
Well, if you've got a bunch of people in a cross-sectional study

如果你们在横向研究中
studied one point in time;

对一个群体进行了一次性研究
they're all going to have different family histories

他们都有不同的家族史
and you've got to measure that, control for it,

你就得衡量或控制这个因素
or whatever.

反正得做点什么
If you're making--using people as their own controls over time.

如果你的实验对象能成为自己的对照标准
The individual's family history isn't changing

个体家族史不变