Both. They look at individual claims, not just accident rates including the configuration you fly it in (skis, floats etc). Except it's not the insurance companies themselves, it seems that it's the underwriters, of which there are only about a dozen nationwide for aviation insurance policies according to a recent seminar I went to. It's an important distinction in places Alaska where many things that would qualify as accidents go unreported in the NTSB database but end up on an insurance adjuster's desk. The exact formula seems to be a closely guarded secret sauce though, no one wants to divulge exactly how the premiums are calculated. I don't know if the info could result in age discrimination lawsuits or something, but it's definitely not something they want to share with the general public.