An analysis of seven years of monthly charge-card sales and tip data from a multi-regional restaurant chain in the United States found that tip percentages predicted food sales in the following month. Thus, restaurant executives, managers, and owners are encouraged to add tip percentages to their sales forecasting models.
Lynn, M., & Ukhov, A. (2013). Tips predict restaurant sales [Electronic article]. Cornell Hospitality Report, 13(8), 6-10.