Predictability of cost, time and quality in design–build construction contracts – evidence from 100 public projects

The study examines the predictability of cost, time and quality in public construction projects delivered through design–build (DB) contracts, employing a mixed-methods approach that combines quantitative analysis with interviews. Based on data from over 100 completed DB contracts, the study assesses schedule adherence, cost deviations and the DB model’s impact on project quality. The research finds that infrastructure contracts demonstrate a schedule predictability with nearly 75% completed on or before the agreed deadline. However, there is substantial variance in the data set, with actual completion times ranging from one year early to more than two years beyond the deadline. The data from building contracts showed significantly less overall variation but were more prone to modest delays, with two out of three contracts completed one to three months late. Regarding costs, the average increase from the original contract amount was 14%, with a median of 10%. Approximately half of the contracts had change volumes of 10% or less, and around 80% fell within a 20% range. Regarding quality, the findings suggest that DB contracts often encourage delivery to minimum standards; therefore, their use should be approached with caution in projects requiring high degree of quality predictability but with poorly defined requirements.