Alternate suggestion: Have a textbox that says, "If you don't mind us asking, where did you hear of us?"
Then the radio ad or whatever can say, "Enter DAN SAVAGE as your referrer to receive a free gift!"
Then the radio ad or whatever can say, "Enter DAN SAVAGE as your referrer to receive a free gift!"
People might voluntarily divulge who referred them to your store. Only with the correct entry do you get a discount, as an added bonus to people who answered.
So incorrect results arn't technically incorrect, they're just bonus data.