Incomplete Contracts in Multi-period Procurement
研究了买方在长期项目中面对供应商成本随世界状态变化时，不完全合同是否构成问题。发现若状态公开且不能更换供应商，不完全合同与完全合同效果相同；否则不完全合同会带来问题。对采购顾问的调查证实了这些条件在实践中的重要性。
We analyze the problem of a buyer who purchases a long-term project from one of several suppliers. A changing state of the world influences the costs of the suppliers. We distinguish between complete contracts conditioning on all future realizations of the state of the world and incomplete contracts renegotiated whenever the state of the world changes. We provide conditions such that incomplete contracting does not pose a problem. If the changing state of the world is publicly observable and the buyer cannot switch between suppliers during the lifetime of the project, the buyer achieves the same surplus irrespective of whether contracts are complete or incomplete. An English auction followed by renegotiation whenever the state of the world changes is optimal. To identify conditions when buyers should consider drafting complete contracts, we extend the analysis by considering private information about the changing state of the world and supplier switching. In both cases, incomplete contracting poses a problem. In a survey of procurement consultants, we confirm that publicly observable states of the world via price indexes play an important role in procurement. Moreover, the consultants confirm that supplier switching is infrequent in procurement practice. Thus, incomplete contracting is less of a problem in a considerable share of procurement projects. However, complete contracts are useful and could be used more often. This paper was accepted by Yan Chen, behavioral economics and decision analysis.