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Flexible software development models, e.g., evolutionary and incremental models, have become increasingly popular. Advocates claim that among the benefits of using these models is reduced overruns, which is one of the main challenges of software project management. This paper describes an in-depth survey of software development projects. The results support the claim that projects which employ a flexible development model experience less effort overruns than do those which employ a sequential model. The reason for the difference is not obvious. We found, for example, no variation in project size, estimation process, or delivered proportion of planned functionality between projects applying different types of development model. When the managers were asked to provide reasons for software overruns and/or estimation accuracy, the largest difference was that more of flexible projects than sequential projects cited good requirement specifications-and good collaboration/communication with clients as contributing to accurate estimates.