Recall our car fuel usage example in which the question is: "What fuelusage should Iexpectfrom this car?" Let "y" be the fuel use, which depends on "xa",fuelvolume consumption, and "xb distance traveled. We determine y by varying these twovariables(that is, we drive the car). Butthe test result can be affected by discrete extraneous variables such asthe route, driver, weather, and road conditions, For example, driving only on interstate highways would impose a false (untypical) trend on our intended average fuel estimate, so we could drive on differenttypes ofroadsto break up this trend. This approach introduces a random test strategy.