Earlier, Pilotgil wrote:

...I am a graduate student conducting research
on student pilots and flight schools.

Have you ever been 'fed-up' with a flight school
and quit? Lousy experience? Bad CFIs? Poor
reliabilty? Took all your $$$... I need to survey
YOU!...

Let me get this straight: You only want to survey aviation
students who are ****ed off? What about all of the
students who are perfectly happy with what they've
found, don't you want to hear from them?

This is the kind of selection bias they warned us about
in the undergrad psyc class on tests and measurement.
I'd think a grad student would know better. Unless,
of course, the test is designed specifically to measure
selection bias...

Bob 'Man is the mismeasure of all things' K.