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.
とても単純に 多くの人を集め 半分に分け 一群をある方法で 残りは別の方法で手当し しばらく経ったら 追跡調査で様子を調べます