Forecasting the success of megaprojects, such as the Olympic Games or space exploration missions, is a very difficult and important task because of the complexity of such projects and the large capital investment they require. Megaproject stakeholders do not typically employ formal forecasting methods, relying instead on Impact Assessments and/or Cost Benefit Analysis; these tools do not necessarily include forecasts, and thus there is no accountability. This study evaluates the effectiveness of judgemental methods towards successfully forecasting the accomplishment of specific megaproject objectives – when the measure of success is the collective accomplishment of such objectives. We compare the performance of three judgemental methods used by a group of 55 semi-experts: Unaided Judgement (UJ), semi-Structured Analogies (s-SA), and Interaction Groups (IG). The empirical evidence reveals that the use of s-SA leads to accuracy improvement compared with UJ. This improvement is amplified further when introducing pooling of analogies through teamwork in IG.