Even if the study's methods are spot-on and there's nothing in there for you to get suspicious about, asking who the data came from is important as you evaluate its conclusions. Let's say the macular softening research collected participants on a college campus. They got 1,000 subjects to study, which isn't too shabby. But because it was a college campus, their participants skewed young (students) and middle-aged or older (professors in their fifties and sixties). In this case, we'll imagine the breakdown is something like 900 college students and 100 adults, almost all of whom are over the age of 40. A thousand subjects sounds like a nice, reasonably high number, but it turns out only 100 people represent the demographic that's reportedly less likely to have phone-related macular softening. This makes it difficult to know whether they've actually found a difference between younger people and older people with regard to macular softening, or if they've merely shown it's a common phenomenon among college students—their sample of older subjects is too small to be sure it's not randomly skewed by a few unusual subjects. The scientists also admit that, due to the college's demographics, their participants are 90% American and 90% Caucasian (and 100% enrolled in an expensive Ivy League school). This makes it impossible to draw conclusions about young people as a whole, because the average young person is not well represented in the data.