Localized Competition and the Aggregation of Plant-Level Increasing Returns: Blast Furnaces, 1929-1935
研究大萧条时期高炉行业工厂级数据，发现尽管工厂间差异很大，但工厂级关系仍能加总为行业级关系，原因在于短期内不同工厂的产出难以相互替代。
A recent empirical literature has shaken economists' confidence in the value of aggregate (industry-level) data to illuminate production relationships. But the statistical finding 'you can't aggregate, ' however well documented, is not an economic explanation. Plant-level relationships do aggregate in Depression-era blast furnace operations despite the presence of very substantial interplant heterogeneity, the most common economic cause of nonaggregability. The economic explanation of this lies in poor short-run substitutability of one plant's output for another's. Substitutability determines the importance of composition effects in understanding aggregate time series, constrains the potential cleansing effects of recessions, and therefore influences industry evolution quite broadly. Copyright 1996 by University of Chicago Press.