Direct versus Implicit Superlative Index Number Formulae

ECONOMISTS and statisticians who construct estimates of total factor productivity or who estimate production functions or systems of consumer demand functions are often forced to aggregate subsets of their data. In order to perform this aggregation, an index number formula is generally used. A price index P(pO, pl, x?, xI) is defined to be a function P of the prices of the N commodities to be aggregated in periods 0 and 1,p?-(pll, . . . , PNO) and pl (pl,.'.. PN'), respectively, and of the corresponding quantities utilized during periods 0 and 1, x? (xi?, . . .,XNO) andX1 _ (xi', . . .,XN1), respectively. A quantity index Q(p0, pl, x?, xl) is defined to be another function Q of the price and quantity vectors for the two periods. Generally, we assume that P and Q satisfy Fisher's (1922) weak factor reversal test: