A parametric model is a set of related mathematical equations that incorporates variable parameters. A scenario is defined by selecting a value for each parameter. Software project managers use software parametric models and parametric estimation tools to estimate their projects' duration, staffing and cost. In the early 1980s refinements to earlier models, such as PRICE S and SLIM, and new models, such as SPQR, Checkpoint, ESTIMACS, SEER-SEM or COCOMO and its commercial implementations PCOC, Costimator, GECOMO, COSTAR and Before You Leap emerged.