How does a randomized block design control for differences among automobile manufacturers?

In a randomized block design, the experimenter controls for differences among automobile manufacturers by grouping the experimental units—in this case, the cars—into blocks based on their manufacturer.1

By ensuring that each treatment (such as a gasoline additive) is tested on an equal number of cars from each manufacturer, the design prevents the results from being skewed by the inherent performance differences of the engines produced by different companies. Because the comparison of the treatments occurs within these homogeneous blocks, the influence of the manufacturer is effectively neutralized, allowing the researcher to isolate the specific effect of the gasoline additive on fuel efficiency. This method provides more precise results than a completely randomized design, where chance could lead to an uneven distribution of cars from a superior-performing manufacturer across the treatment groups.1