[colmvp]

"You probably don't need a degree in experimental psychology to understand that forced recruitment gives you invalid/biased/low quality data."

Interesting point. Anecdotally, I am less honest with my selections if I'm taking a survey specifically to get a reward (i.e. contest, Kongregate points, access to a site), as I usually just want to get it done with as fast as possible.

However, if it has tangible results (i.e. will tailor my experience on the site like Netflix) then I am likely to spend more time choosing honestly.

[masterzora]

Even the honesty of your answers is not the only issue. One of the big ones is that the set of people opting in to a survey is already demonstrating a characteristic different from the set of all people of interest unless the people of interest are for some reason "the set of people opting in to a survey". This alone is enough to muddy or even invalidate the output data.

[rabidsnail]

http://en.wikipedia.org/wiki/Self-selection