Measuring Time: Earned value or earned schedule?

In “Earned Value Management: The three key metrics”, it has been discussed that EVM relies on three key input metrics, known as the planned value (PV), the actual cost (AC) and the earned value (EV). In this article, a fourth key metric is discussed which is based on the periodic values of the PV and EV. Figure 1 shows the S-curves for the three key parameters over time, up to week 7. Only the planned value is known from the project start until the end, as a result of the baseline schedule construction. The actual cost and the earned value are based on weekly progress measurements during project execution.

 

 

The fourth key metric, known as Earned Schedule (ES), is a simple translation of the EV of a given status date into time units by determining when this EV should have been earned in the baseline schedule. A formal definition of the ES metric can be given as follows:

 

Find t such that EV ≥ PV_t and EV < PV_t+1

ES = t + (EV - PV_t) / (PV_t+1 - PV_t)

 

With 

 

Example week 7:

 

This value is illustrated graphically in figure 2.

 

 

While EVM measures schedule performance not in units of time, but rather in costs, the Earned Schedule metric, instead, measures your project progress in a time dimension and varies between 0 time units (at the start of the project) and the baseline Planned Duration (PD) at the end of the project. Hence, at the end of the project, EV = PV and ES = PD. In “Earned Value Management: Reliable time performance measurement”, it will be shown that the ES is also used to measure the time performance of a project using an alternative schedule performance index, which is more reliable than the traditional SPI = EV / PV.