Score contribution per author:
α: calibrated so average coauthorship-adjusted count equals average raw count
The optimal duration of a supply contract balances the costs of re-selecting a supplier against the costs of being matched to an inefficient supplier when the contract lasts too long. I develop a structural model of contract duration that captures this trade-off and provide an empirical strategy for quantifying (unobserved) transaction costs. I estimate the model using federal supply contracts for a standardized product, where suppliers are selected by procurement auctions. The estimated transaction costs are substantially greater than consumer switching costs and a significant portion of total buyer costs. Counterfactuals illustrate the importance of accounting for the duration margin.