In this study, I find that the majority of this 'residual' variance is due to stable long-term differences among business-units rather than to transient phenomena. Using Schmalensee's sample, I find that stable business-unit effects account for 46 percent of the variance. Indeed, the stable business-unit effects are six times more important than stable industry effects in explaining the dispersion of returns. Business units differ from one another within industries a great deal more than industries differ from one another.