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Unfortunately for tipping, a number of studies point toward a positive but weak relationship between service quality and the amount tipped. For example, a study by Cornell University Professor Michael Lynn and Ithaca College Professor Michael McCall, combined the results of 13 studies using exit interviews of 2,547 dining parties at 20 different restaurants and found that the correlation between tip percentage and service ratings was only 0.11—that is service quality explained, on average, only a very small proportion of the variation in tip percentages.[4] Also worrying, was that they found that tips were not related to servers’ or third parties’ evaluations of service. Another study by Professor Michael Lynn, this time with fellow Cornell University Professor Ted O’Donoghue and Professor Michael Conlin from Syracuse University, looked at the tipping behaviour of 1,393 dining parties in 39 restaurants in Houston, Texas, and found that one point increase in service quality increased the tip by only 1.5% of the total bill—not very encouraging considering that the study used only a five point scale. The correlation between tip percentage and service ratings was even lower in this study, only 0.07.[5] These results are important as they call into question whether tipping provides strong enough incentives to improve service quality.
Furthermore, some studies find factors that affect the size of a tip which seem to have little to do with the principal-agent problem. For example, one study reports that lightly touching customers when returning change increases the tip size.[6] Beware too the innocent piece of candy that arrives with your bill as it may be a strategy to elicit a higher tip—one study found that customers who received a small piece of candy give larger tips that those who did not; and that the tip size was related to the size of candy.[7] Other peculiar tip-inducing behaviour[8] includes squatting at the table, drawing a smiley face on the bill, forecasting good weather,[9] telling a joke, and wearing a flower in your hair.
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