Attractive waitresses get better tips for poor service than for good service, and they get better tips than not-so-attractive waitresses no matter what quality service they give, according to a recent study.

The numbers went like this: Pretty waitresses averaged 20.3 percent of the bill as a tip when they gave poor service. Less attractive waitresses averaged 14.9 percent for excellent service and 11.9 percent for poor. Overall, attractive waitresses received an 18.8 percent tip on the average, less attractive ones 13.4 percent.

The study, by Joanne M. May of Loyola University, was published in the Cornell Hotel and Restaurant Administration Quarterly. It found that beside quality of service and the appearance of the waitress (the study did not consider waiters), other important variables in tip size include the number of people in the party and whether the bill was paid in cash or by credit card.

The best tippers were solitary diners who paid with credit cards; they averaged 23.7 percent of the bill. Credit card payers tipped slightly better than those who paid cash -- 15.2 percent vs. 15 percent -- and, for no apparent reason, tables of three or five left less per person than tables of four or six.

All this led May to call for more research into the "complex" nature of tipping.But a former waitress now on the staff of American Demographics magazine, which took note of the study, observed: "Waitresses have known for a long time that the best way to get good tips is to look as cute and as helpless as possible. The customers feel sorry for you."