It's very straightforward -- you take a bunch of people, you split them in half, you treat one group one way, you treat the other group the other way, and a little while later, you follow them up and see what happened to each of them.
非常简单－找一群人，分两组 用不同的方式分别对待两组人 之后你跟着他们 看他们都怎么样了