I suspect it’s the lamppost effect: the data was here, so they used that rather than a more relevant observation that would have cost a lot more money. As it’s

I suspect it’s the lamppost effect: the data was here, so they used that rather than a more relevant observation that would have cost a lot more money. As it’s a health-related database, I suspect it’s a photo taken to control for acne, but that’s just speculation. But definitely agree that it’s probably skewing the results a lot.