Experimental Design for Performance of Garbage Trucks

Problem

New York City purchased over a hundred million dollars’ worth of new garbage trucks. Upon fielding the trucks, the city discovered that they were packing fewer tons of garbage than expected.

The manufacturer offered to retrofit the trucks with enhanced hydraulics to upgrade the vehicles' capacity. A special Corporation Counsel for the city asked Analysis & Inference if it could design a test to determine if the fix would be sufficient.

Procedure

In collaboration with a mechanical engineering consultant, A+I designed a field experiment in which a small number of retrofitted trucks made runs and recorded their loads. The trucks were rotated among different areas of the city on different days. A strict protocol required uniform conditions for operating the vehicles, and for measuring the tons packed. Furthermore, no truck could repeat collection on the same day in the same area.

Result

The statistical analysis of the tons collected over the experimental runs showed a classic bell-shaped curve whose mean was comfortably above the spec, and whose 10th percentile was right at the spec. While the contract had not spelled out all the details of what results were required, the two parties agreed that the delivered vehicles were good.