AUSTIN STUDY: MILES-BY-THE-MONTH
The arrangement with participating clients was defined in newspaper ads, such as the pdf file linked here.
This graph shows the high, average, and low number of miles in the month between consecutive odometer readings each client's car was driven.
From left to right the cars are shown by increasing average miles. The letter following each car's number indicates the number of odometer readings after the first: A=1, B=2, C=3 . . . P=16, Q=17, R=18.