There is increased interest in industrial experiments aimed at improving quality by reducing the variability of a product characteristic while maintaining the desired mean level of the characteristic. Analysis of treatment effects on variability, however, is more difficult than analyzing the effects on mean performance. In this article we extend the results of Zelen to test for treatment effects on variance when there are two sources of variability, that which exists between production runs or setups and variability within runs. Bootstrap critical values are developed to handle possibly nonnormal errors at either level of variability.