How large is the Soviet gross national product (GNP) relative to our own? A third as large? Half? Two-thirds? Which of the estimates is correct? As the unique solution, none of them, unfortunately. Nor need we search for other numbers: these are probably the best of the lot, and in any case, the reply would be the same. The problem is not one of data, or of definitions, or of estimating methods. Such problems do exist and present difficulties of their own, but the inescapable and immovable barrier to the unique solution in US-USSR national output comparisons is the crucial fact of differences in the structures of the American and Soviet economies. In general, in comparisons of different economies or of the same economy at different times, diverse structures create what the economist calls an “index-number problem.” The worst of it is, the “problem” is insoluble.
