A nice illustration of the point in the previous post, lifted from the comments to the Kathy Sierra post cited there:
I remember an exercise that swatch did when they first came out; organising a display in a hotel foyer, then picking people off the street to ask which watch design they’d choose if they were getting one that day. The market researcher duly noted the persons choice.
On the way out, they were asked to "just take one watch off the other stand on the way out to keep as a thankyou for participating". Which all did.
The two stands were identical. However, the watches chosen for the research and which were taken were completely different…