Incentive pay systems are a bitch to design, because people are so good at gaming them. One call centre I worked at had contests on Friday if weekly sales had been down. One woman, no matter how bad she was doing by lunch, always pulled out a bunch of sales in the afternoon, and was given the $50 cash prize.

I was the data analyst, and got suspicious. Sure enough, all most of those 'sales' would be cancelled on Tuesday or Wednesday of the next week. We listened to the tapes of her sales calls, and heard "I'll put the order in to reserve your spot; if you change your mind over the weekend, you can cancel." She didn't deny any of this when confronted with the evidence; she also didn't come back to work the next day, or ever after.

Also, the top performers on each team seemed to win week after week, which makes sense. They are the best sales people, so they generally sell more at all times. But an incentive that goes to the top performers most of the time only reinforces an "us vs them" mentality on the sales team, where the top performers are seen as getting the best leads (they do), and getting the most slack for things like being late, etc. (they did). As I said, designing a good, fair, working incentive system that can't be 'gamed' is not easy.