Doing a study which polls how many Republicans watch Fox News is not any different than a ratings system which polls how many people watch Lost. If one is considered a study, so is the other.
Are you being purposely dim?
No, one "study" is not as good as another. It depends on the conditions under which the study was conducted, the historical accuracy of the firm conducting the study, the parameters of the study itself.
The Nielsen ratings utilize a combination of electronic monitoring devices and paper surveys to determine a representative quantity of viewers for a particular television show. The ratings system is not designed, nor does it intend, to determine the value or position of the shows. It is a tabulation system more than a survey and as such is not nearly as likely to be influenced by the personal bias of the person(s) managing the system. The Nielsen ratings also have a long track record of relative accuracy.
The moment you introduce personal bias into the equation, you have a completely different animal. It's ignorant to pretend otherwise. If an interviewer asks you "Do you watch Psych on USA?" Your answer is a simple yes or no. It's not subject to any other factors. If an interviewer asks you to identify your political preference, any answer you give thereafter will be treated differently.
A poll that tells me how many people watch Fox News has the chance to be objectively measured. One which purports to tell me how many Republicans watch the same channel is hopelessly flawed.
I can tell you how many people attend a game at Jordan Hare Stadium on Saturday. Identifying how many attend because they like the color orange, because they enjoy watching the eagle fly, because they are fans of the opposing team, etc -- all subjective and impossible to accurately quantify.