A Capacity-Aware Parr Model for Agile Projects
Classical software effort distribution models, including the PNR family and Parr alter native curve, were designed to describe the time distribution of development effort under an implied staffing pattern. Their direct use in agile environments is limited when team capacity is fixed, partially fixed, or externally constrained, the original curve may prescribe a staff demand that the organization cannot allocate. This paper proposes a compact refactoring of Parr model as a capacity-aware forecasting layer for agile projects. The contribution is deliberately narrower than a full causal theory of project dynamics. A normalized Parr shaped latent effort demand is combined with an observed or planned capacity trajectory. The resulting model forecasts aggregate progress, completion time, capacity deficit, and capacity slack without assuming that the same internal activity path is followed under resource restriction. The model uses a small parameter set such as total effort K, a Parr shape parameter, an origin constant c that can match nonzero initial staffing, and the capacity trajectory. A discrete sprint formulation is provided, together with a calibration method from ordinary Scrum records and a rolling origin validation protocol against simple management baselines.