Clearly, you are having trouble reading...

"LOL. EVERY SINGLE ENTITY has some differences from each other. You're doing the same thing the other poster did. You're basically saying EVERY study that samples is invalid because they don't count every single item in the population."

Wrong. I said...

"they didn't even sample an entire category of low-wage employers"

What part of the difference between "category" and "every single entity" are you having difficulty understanding?

Legitimate statistical analysis requires samples from all the known categories of data relevant to the study, that is NOT "every single entity".  This shouldn't be difficult to understand.