IN THE ANALYSIS of economic data the investigator is frequently provided with only the simple sum of some elementary variables. The elementary variables are known to be perfect substitutes (i.e., to have the same effect) in some contexts, but not in all the relevant ones. Familiar examples are hours of work at possibly different jobs, hours of leisure spent at different activities, number of trips abroad, cigarette consumption (of possibly different brands). The question, naturally, is under what conditions can we expect that such aggregates will satisfy the same restrictions as those imposed by the theory on the elementary variables? As an answer we suggest the following variation of Hicks' [3, p. 33] composite good theorem: A simple sum of elementary goods can be treated as a single good if their (properly normalized) prices vary so as to keep all within group differences constant.2 We shall illustrate the proposition in the context of standard demand theory. Define the expenditure function: m (u, p) minm 1 l7pixi, subject to u(x1.. .xn) = u. Suppose that

[This paper is concerned with the sources of variation in the earnings of American scientists over the decade 1960-70. It focuses on sources of variation over time as well as at a point in time. Earnings variation over time is decomposed into sources due to measurable variables representing an earnings function, a random effect individual variance component in the level of earnings, a random effect individual component in earnings growth, and a serially correlated transitory component. Maximum likelihood estimates of the implied parameters are presented and a method for obtaining GLS estimates of the earnings regression function, when the residual variance components are given, is explored.]