The Limits of the Linear Approach to Schedule Variance
In earned value management (EVM), the deviation from the project schedule is represented with the Schedule Variance (SV) indicator. It helps the project manager to notice the deviation and to trigger corrective actions.
The authorized work that should have been accomplished at some point in time is represented by the Planned Value (PV) variable. The Earned Value (EV) variable corresponds to the work that has been completed. The difference between EV and PV is equal to the SV. The ideal project has an SV equal to zero all along its duration. So, any non-zero values of SV indicate that the project is ahead or behind schedule.
For further analysis, we’ll take the budget at completion (BAC) as equal to 1 regardless of the actual units used in practice. Also, we assume that both PV and EV can be determined at any moment of time during the project.
The Planned and Earned Value Model
The PV distribution in time serves as a guideline for the schedule performance monitoring. The sigmoid (logistic) function is taken to model the PV (the middle of the project is the most intense in terms of work to be done). For EV, we’ll take the quadratic function, which models the project that runs behind the schedule most of the time. The chart below depicts the PV and EV variables:
The X-axis represents the time variable, which changes from 0 (start of the project) to 1 (
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