Because you could have these confounding variables. Now experiments, and experiments are the basis of the scientific method. Experiments are all about trying to establish causality. And so what you would do is, if you wanted to do an experiment, you would take, and you probably wouldn't be able to do it with a thousand people, experiments in some way are the hardest to do of all of these. Maybe you take a hundred people. A hundred people. And to avoid having this confounding variable introduce error into your experiment, you randomly assign these hundred people into two groups.