"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.
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.
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.