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A plethora of estimation methods exists and there are several schemas for classifying them. Here we classify estimation methods in "expert estimation" and "formal estimation models".

- "expert estimation" is used as a label for estimation methods in which a significant part of the estimation process (particularly the final step, i.e., the “quantification step”) is based on intuition. Expert judgment-based estimation is not a single estimation method, but a spectrum of judgment-based processes. Even within the same software development organization, processes of expert judgment-based estimation may vary from those based on pure intuition to highly structured processes that use relevant historical data.

- "formal estimation models" is used as a label for estimation methods where a substantial part of the estimation (and particularly the "quantification step") is based on the use of mechanical and analytical processes, e.g., the use of a formula derived from historical data using regression analysis.

Software effort estimation research is inconclusive regarding which estimation approach is better, e.g. a recent review of studies comparing models and experts in software development effort estimation concludes that experts typically performs no worse than the models. One reason for this may be that it seems to be difficult to develop meaningful estimation models that do not require a high degree of expert judgment as input to the models in the first place.

Papers on estimation by estimation method (Links not currently supported):

 

See also the Methodology Tree in Principles of Forecasting: A Handbook for Researchers and Practitioners.