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We use transaction-level US import data to compare firms from virtually all countries in the world competing in a single destination market. Guided by a simple theoretical framework, we decompose countries’ market shares into the contribution of the number of firm-products, their average attributes (quality and efficiency) and heterogeneity around the mean. Our results show that the number of firm-products explains half of the variation in sales, while the remaining part is equally accounted for by average attributes and their dispersion. Quality is the main driver of firm heterogeneity (explaining between 75% and 100%). We then study how the distribution of firm-level characteristics varies across countries, and we explore some of its determinants. Countries with a larger market size tend to be characterized by a more dispersed distribution of firms’ sales, especially due to heterogeneity in quality. These countries also tend to be more likely to host superstar firms, although this is not the only source of higher heterogeneity. To further explore the role of exceptional firms, we develop a novel decomposition that separates the contribution of heterogeneity from that of granularity. While individual firms matter, we find that heterogeneity is more important than granularity for explaining sales.