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Reliability and Accuracy of Real-Time Visualization Techniques for Measuring School Cafeteria Tray Waste: Validating the Quarter-Waste Method

Hanks, Andrew S., David Just and Brian Wansink. (2014). Reliability and Accuracy of Real-Time Visualization Techniques for Measuring School Cafeteria Tray Waste: Validating the Quarter-Waste Method. Journal of the Academy of Nutrition and Dietetics, 114(3), 470-474. doi:10.1016/j.jand.2013.08.013

Shortly after the new National School Lunch Program regulations came into effect in the 2012-13 school year, many schools reported that students were wasting a larger portion of foods purchased resulting in an increase in food waste. Many schools have been under pressure to measure tray waste in order to see whether lunchroom interventions are impacting student consumption of foods. Thus far, the most accurate tray waste method consists in individually weighing leftover food, however, this method is not always feasible because of its relative higher cost and the time spent collecting data. We conducted this study to test three methods of visual waste measurement, the half waste method, the quarter waste method and the photography method to find out which method is the most reliable, accurate, and cost effective, relative to weighing each tray.

The study was conducted in an elementary school of kindergarten to 5th grade students. Tray waste was collected for all 197 full lunches purchased the day of the study. Researchers weighed 5 complete servings of each food to get an average weight per serving and to familiarize them with what one portion of each food looked like to accurately estimate how much of that portion remains on a tray once the student is finished eating. Students were instructed to leave their trays with uneaten food on the tables at which the researchers sat. When a tray was received, it was assigned a number, and then leftovers were measured using each of the visual measuring methods before being weighed on a scale. For the photograph method researchers simply took a photograph. For the half-waste method they recorded whether none, some, or all of the food remained and for the quarter-method, researchers recorded whether none, ¼, ½, ¾, or the entire food portion remained on the discarded tray.  For packaged items, such as milk cartons, researchers estimated the amount left inside the package by holding the item in their hands.

A reliability score was calculated for each of the three methods by comparing correlation coefficients of amounts wasted for each of the visualization methods to the actual weight of the wasted food. The quarter waste method had the highest reliability rating: 90%. The half waste method had 83% reliably and the photograph method was 48% reliable. The low rating of the photograph method is likely due to the inability to estimate remaining amounts of packaged foods. Next the methods were compared to see which provided the most accurate data in terms of linear difference. There was great variability between the accuracy of measurements for different food items; however, the photograph method proved to be the least accurate of the three methods. For each tray the quarter and half waste methods took 4-5 seconds and the photograph method took about 6-7 seconds.

Based on these findings researchers concluded that the quarter waste method is the most reliable visual method, and is as accurate as the half-waste method and more accurate than the photograph method. The quarter waste method is also cost effective, simple to implement and requires little time to enact. As such, it is the best visual method for cafeteria managers who need to know the types and amounts of food being thrown away.