What are the most frequent routes that patients find their primary
care doctors, and what's the percentage breakdown?  Does this vary by
geography (e.g. by state/city)?

An acceptable answer has to go beyond just something like:
50%: look in their insurance directory
30%: have a relative/friend who uses the doctor

but more like:
50$: look in their insurance directory
>> of those 50%, they select from the list in the directory by
>> 40%: asking a friend if they've heard of them
>> 30%: looking at their training (e.g. Ivy League medical school)

etc.  Or, part of the answer can look like a market research report:
e.g.
Respondents rated these aspects as most important in selecting a new
primary care physician:
- Speaks my language : 9/10
- Takes my insurance: 9/10
- My friend said they're good: 8/10
- Location : 7/10
- Expertise : 6/10
- Bedside manner : 4/10

etc...

but I particularly want to know not just what they want out of a
doctor, but how they go about developing their list, and then
ultimately selecting one:
e.g. I looked online (30%)
I asked a bunch of friends (90%)
I read "Best Doctors of X City, 2005" (15%)