To add some hydrogen to this particular fire....
Suppose you doctor does ask if you own firearms, and your answer is to say "none of your business" or to not answer at all. How do you know that you are NOT being put in the category of a firearms owner? It could well be that any answer other than a clear "no" would put you in that category!
It has happened before with othe statistics. Race, for one. People who have not answered clearly on a "which race" question have been statistically "assigned" a racial group.
Or so I have heard........
