Unit effects in software project effort estimation: Work-hours gives lower effort estimates than workdays

Context: Software development effort estimates are typically expert judgment-based and too low to reflect the actual use of effort. Objective: To understand how the choice of effort unit affects expert judgement-based effort estimates, and to use this knowledge to increase the realism of effort estimates. Method: Two experiments where the software professionals were randomly instructed to estimate the effort of the same projects in work-hours or in workdays. Results: In both experiment, software professionals estimating in work-hours had much lower estimates (on average 33% 59% lower) than those estimating in workdays. We argue that the unitosity effect—i.e., that we tend to infer information about the quantity from the choice of unit—is a good candidate explanation for the large difference in effort estimates. Conclusion: A practical implication of the unit effect is that, in contexts where there is a tendency towards effort underestimation, the instruction to estimate in higher granularity effort units, such as workdays instead of work-hours, is likely to lead to more realistic effort estimates.