Why do you let people with few items ahead of you in the paying line of a supermarket? Presumably it is more annoying to wait when you know you will take a short time to do your transaction. At what point do people decide to let someone ahead of them?
It would seem to be based on
1. Number of items they have
2. Number of items the person behind you has
3. The length of the queue. It seems that if the queue is long letting someone ahead of you is of less marginal return.
It is not in the supermarkets interest that queue skipping happens. Bulk purchases take less time for the amount of goods then small purchases. As in each item takes a second and the paying bit takes 30 seconds so you would like lots of items and few payments. So shops would prefer big purchases. Letting people ahead of you rewards small purchases by reducing the waiting time for them which supermarkets should want to discourage.
The must be some generally accepted graph of when it is expected for you to let someone ahead of you. If you have any evidence of when people are let ahead in a supermarket queue please comment.