The difference is that by taking data from a dating site, you're restricting yourself to the intersection of "uses dating site" and "is/is not atheist." In ord

The difference is that by taking data from a dating site, you're restricting yourself to the intersection of "uses dating site" and "is/is not atheist." In order to argue that these people are representative of their self-reported religious affiliation, you have to show that the "uses dating site" subset is representative of the whole.