According to studies published by Professor Michael Lynn, an expert on tipping at the Cornell University School of Hotel Administration, tip percentages are only weakly related to customers’ ratings of the service quality they received. For restaurant diners in particular, other factors influencing tip percentage include the size of the dining party, whether or not the customer pays by credit card, and the sex of both the customer and the waiter. Lynn’s research found that men left larger tips than women in cases where the server was female, and women tipped more than men when served by a male.