let's say have people lose weight,

比如说让他们减肥
eat more fruits and vegetables,

让他们多吃水果和蔬菜
take supplements, whatever it is.

服用营养补充剂什么的
Then you follow people--that same group of people over time

然后对这组人进行长时间的跟踪观察
and you can get a pretty strong cause and effect inference out of that.

你们从中会发现一个紧密联系的因果关系
The reason you can draw a stronger inference is

你之所以能得到强有力的推论
in part is because the people, as I said,

部分原因是如我说的
are their own controls,

人们是自己的对照标准
and so variability that gets introduced ito studies

所以这个研究方
because of differences between people

排除了
don't exist using this method,

个体差异造成的易变性
because the same group of people are followed over time.

因为一直都是对同一群体进行追踪