Why project size matters for contract choice in software development outsourcing

The contractual mechanism of software development outsourcing, typically either fixed-price (FP) or timeand- materials (T&M), determines the nature of incentives, risk sharing, and coordination between client and vendor. While software engineering considers project size as crucial for project planning and success, neither economic nor organizational theory considers size per se among the determinants of contract choice. In this paper, we address the gap between the centrality of project size in the software engineering literature and the attention it receives in software contracting research by modeling and testing the association between project size and contract choice. Existing empirical evidence indicates that FP contracts are appropriate for small development efforts whereas T&M contracts are suitable for larger projects, based on the reasoning that cost and schedule are difficult to estimate in larger projects. This prediction that size is directly associated with contract choice is the basis upon which two models are developed. The first model draws on the contracting efficiency approach to hypothesize that the effect of project size on contract choice is mediated by project detail. The second model draws on the contingency approach to software development risk management to hypothesize that the effect of project size on contract choice is moderated by project detail and vendor familiarity. We test these models using a large portfolio of software development contracts entered into by a leading European bank, and the results confirm that both mediation and moderation are at play.