DrJack wrote
So I found it interesting that in a
recent poll of BLIPMAP users, of the 430 respondents who listed
themselves as a "Sailplane" pilot the percentage who also said they
were SSA members was _91%_.

I think the main selection bias you are missing is the low
participation of power pilots. Less than 30% of the respondents were
power rated at all; given my experience with active soaring pilots who
are also power rated, I would guess that means that only about 10-20%
of the respondents are active in power flying.

Further, I think of the soaring pilots not represented by SSA, the
majority are primarily power pilots with limited soaring interest.
After all, if you're going to get badges, enter constests, or be in a
club, you generally have to join SSA.

there would have to be a huge opposing bias in the
non-participating pilots to get down to a 50% overall SSA-member
level.

Actually, I think the real selection bias is being a BLIPMAP member in
the first place. I never even heard of it until now, and I'm
primarily a power pilot and occasional soaring pilot who is not an SSA
member.

FWIW, my conclusion is that the number of _active_ soaring
pilots who are also SSA members is much greater than 50%

Almost certainly, especially if by 'active' you mean those who soar
most weekends. An active power pilot is going to have other interests
(mainly power flying) and is unlikely to soar as much as someone whose
only flying is soaring.

so the pool
of non-SSA soaring pilots which might be used to increase SSA
membership is not nearly so large as Roake's statement would suggest.

Actually, I suspect that those soaring pilots who are not SSA members
are mostly not available to increase SSA membership at all. Someone
whose primary interest is flying, and who considers soaring to be just
one aspect of it (and not the most important one at that) is much more
likely to give his money to AOPA or EAA.

My experience is that there are lots of inactive and sporadic soaring
pilots out there who are active in other kinds of flying. Some just
got the rating with no intent of ever using it (should they be called
gliding rather than soaring pilots?) but many others actually have
some XC experience, and don't soar much (or at all) because soaring is
so inconvenient compared to powered flying. Good luck trying to
convince those people that they should send money to SSA.

Michael