It's just that 1. Sampling and self reporting introduces many concerns to the validity of the study. Let's not downplay the negative effect these can have. 2

It's just that 1. Sampling and self reporting introduces many concerns to the validity of the study. Let's not downplay the negative effect these can have. 2. It's expensive to collect consent and data. 3. Federal grants often fund these studies. Tax payers are essentially paying to collect the same data twice.