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.
Vrlo je jednostavno -- uzmete hrpu ljudi, podijelite ih u dvije jednake grupe, jednu grupu tretirate na jedan način, drugu na drugi način, vratite im se nešto kasnije i vidite što se dogodilo sa svakom od njih.