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 cannot 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.

We analyse both initial underpricing and post-listing returns for Australian IPOs. Our results are consistent with the view that unique institutional characteristics may have overwhelmed previous Australian tests of equilibrium models of IPO underpricing. The results also show that Australian IPOs significantly underperform market movements in the three-year period subsequent to listing. Further investigation of these anomalous post-listing returns lead us to reject various ‘speculative bubble’ explanations. Rather, the evidence suggests a curvilinear relationship between initial and subsequent returns, although the economic significance of the relationship is low.

This paper attempts to reconcile the contradictory findings in the debate over school resources and school effectiveness by highlighting the role of aggregation in the presence of omitted variables bias. While data aggregation for well-specified linear models yields unbiased parameter estimates, aggregation alters the magnitude of any omitted variables bias. In general, the theoretical impact of aggregation is ambiguous. In a very relevant special case where omitted variables relate to state differences in school policy, however, aggregation implies clear upward bias of estimated school resource effects. Analysis of High School and Beyond data provides strong evidence that aggregation inflates the coefficients on school resources. Moreover, the pattern of results is not consistent with an errors-in-variables explanation, the alternative explanation for the larger estimated impact with aggregate estimates. Since studies using aggregate data are much more likely to find positive school resource effects on achievement, these results provide further support to the view that additional expenditures alone are unlikely to improve student outcomes.