You're misusing the word "factor".

Regulation is always a factor in the United States. Whether it is a significant or insignificant factor is the question. Regulation is historically in flux, and local, which is why it can't be controlled for. And why would it be, besides?

Perhaps I misapplied the word "constant". What I meant to say is that theoretically there could be a varying formal measurement, but that this is impractical for the reasons stated.