I agree that no matter how imprecise the methodology of focus interviews
might be, the process gives worthwhile feedback to the museum from its
visitors.

We do them from time to time (one is currently underway), generally trying
to determine  if they felt their visit was 1) worthwhile, 2) memorable, 3)
was the staff helpful and 4) how did they learn of us?   We invite their
criticisms, suggestions, and also make sure they focus on our permanent
exhibits, our changing exhibit, our outdoor settlement, and our museum shop
as they respond to questions.  Would we make changes in an exhibit based on
this approach?  Only if repeatedly, someone found it lacking or of no
interest.

We're in a rural area, but we attract people from every state, and I'm glad
to note how many times we have been recommended by a neighbor, friend, or
family member.  And not so surprised that our marketing materials seem to
draw relatively few families.

Thankfully I've had no experience with having to "prove" to a granting
agency that a museum experience was able to create a very specific change
among a particular population group.  That said, we have had great success
(in most cases) in our work with at-risk or in-trouble young people who are
assigned to us (one or two at a time) through the school system for
after-hours work.  We provide a positive learning experience, a healthy
work environment, a caring group of staff, and the chance for these young
people to show what they can really do if given the chance.  The schools
are rule-driven, and we are not.

As to museum marketing methodology in general?  I also doubt it is truly
possible to
"position" ourselves as do Ralph Lauren and Campbell's Chicken Noodle Soup.
We are not in the trend-setting and brand identity businesses, and
shouldn't be, in my opinion.