The birth rate in the us. is going to be weig down to the tune of like 15 to 20%. That's massive for a year over year change, even though we're all locked up inside. We are apparently doing other things, watching netlicks. Allie gets an a plus for intellectual curiosity in her econometric class. But with that exception, you can think of it as being exogenous. The challenge is not only that you care that it was random when it happened, but also that it doesn't have any influence on the hin that you're going to study that you're not accounting for.