A few months ago BusinessWeek ran an article on how much people tip New York cab drivers. There are bumps in 20%, 25%, and 30%, which is expected because those are preset values in the payment interface. However, a much lower percentage of people tipped 19% than they did 21%. Peculiar.
Ben Wellington, a visiting statistics professor at Pratt Institute, did some digging. It turns out it’s not some glitch in human generosity but a difference between two payment softwares used by New York cabs. One calculates tip value taking a percentage of fare plus taxes and tolls. The other only calculates based on the fare.