And to avoid having this confounding variable introduce error into your experiment, you randomly assign these hundred people into two groups. So random assign. So random. It's very important that they're randomly assigned. And that's nice, you might not know all of the confounding variables there, but it makes it likely that each group will have a same amount of people with lack of activity, or that the lack of activity, or the activity levels on average in each of the groups, when they're randomly assigned, it gives you a better chance that one group doesn't have a significantly different activity level than the other. And then what you do is, you have a control group, and you have a treatment group. Once again, you've randomly assigned them.