Earned schedule analysis is a new concept developed by Walt Lipke in 2003. The Earned Schedule (ES) is now an appendix in the PMI Practice Standard for Earned Value Management and is considered an extension of EVM practice. Although the knowledge and use of ES has grown immensely over the last decade and has spread worldwide, the method remains unknown to many EVM users.
The earned schedule calculation
The Earned Schedule: ES (Earned Time Value)
The concept of the earned schedule is analogous to the earned value. However, instead of using cost ($/€) to measure on-time delivery, time is used. Rather than just looking at on-time delivery based on the $/€ value of work done vs. schedule, the earned schedule looks at it When the work should have been completed. The Earned Schedule method allows us to calculate time-based indicators.
For a better understanding, we illustrate the whole thing in the following section:
To measure the earned schedule, we need to know “when” the work should be done.
In other words, at what point in time would the current Earned Value (EV) have been achieved? This can be in the past or future depending on the EV to PV ratio!
This is determined by drawing a horizontal line from the current cumulative EV to the PV curve. The intersection of this line with the PV curve is then projected onto the time axis. The distance from this point in time to the start of the PV curve then corresponds to the earned schedule. Please see the ES value in the graph below.
The earned schedule shows the planned duration of the work done so far (earned value) at the current time AT (actual time). In our solutions, we use “days” as the unit for the ES. In the graphic below, the ES ≈ 95 days.
The Actual Time: AT
The other new parameter is the current duration (AT), which is the number of days that have elapsed since the project started. In our example, the total number of days executed from the start of the project is ≈ 125 days. Therefore, the current duration (AT) is 125 days.
The planned duration: PD (Planned Duration)
The planned duration (PD) is the “original duration of the project”. The PD in the example is 8 months. Now we have 3 key elements to assess schedule status in units of time:
- Earned Schedule – Earned Time Value (ES)
- Actual Time – Current Duration (AT)
- Planned Duration (PD)
Key ES KPIs
Similar to the traditional EVA metrics, new KPIs can be generated for the TARGET-PLAN alignment of the plan.
- SV(t) = ES – AT
- SPI(t) = ES / AT
To distinguish between the traditional EVM metrics and the earned schedule metrics, a “t” is added for time in the earned schedule metrics.
The important observation is that SPI(t) & SV(t) have remained good indicators of project status even after project completion, while SPI(€) & SV(€) end up at 1.0 regardless of late project completion.
The ES approach therefore has a “memory” effect, which is very helpful in project reporting and subsequent retrospectives.
In addition to the schedule performance metric, similar to the cost forecast metrics, schedule forecast metrics can also be used to predict the schedule completion date (Estimated Time at Completion (EAC(t)) and Variance at Completion (VAC(t)).