Expert Estimations
A proven method to approximate the costs for implementing a user story is the expert estimation. One or more experts independently estimate based on previous experience with similar tasks. Since individual experience can differ to a high degree and depends on the own capabilities, it is obvious to use the average over all estimations. In addition to this subjectivity, another problem of the method is that experts mostly are members of development teams and cannot productively contribute to the development process while performing their estimation.
A derivative of the expert estimation is the Delphi method, where a moderator first presents details of the user stories to be estimated to all involved experts. Subsequently, they perform an individual estimation, completely independent from each other. Then the moderator evaluates, presents the results and thereby points out discrepancies – all with respect to anonymity. Discussions are undesirable to prevent an influence on group dynamics through dominant experts. Every expert has the opportunity to rethink and change his estimation. This process is repeated until the discrepancies do no longer exceed a tolerance threshold. An advantage of the Delphi method is the iterative refinement of all expert estimations and the resulting higher accuracy. A disadvantage is the high time commitment of the experts.
Alternatives:
Indirect Estimations with Story Points
Indirect Estimations by measuring the Functional Size
Book recommendation: "Cost Estimation in Agile Software Development"
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