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Software development effort estimates are often inaccurate. This study investigates to what degree and why the sequence in which we estimate software work affect the effort estimates. The results may be used to improve judgment-based software development effort estimation processes. Two controlled experiments were conducted. In the first experiment, software professionals were randomly allocated to the groups SMALL and LARGE. First, those in group SMALL estimated the most likely effort required to complete a small software development task and those in group LARGE the effort of a larger task. Then both groups estimated the effort of the same medium-sized task. The first estimate had a large impact on the subsequent. The second experiment aimed at a better understanding of the nature of sequence effects in effort estimation. This experiment suggests that it is the experience and knowledge activated in the previous task that matter, not the estimated value itself. In conclusion, more awareness of the importance of the estimation sequence may lead to more realistic effort estimates. In particular, it may be useful to avoid estimation of simple and small tasks just before larger and more complex tasks in situations where over-optimism is frequent.