Measuring Time: Earned value or earned schedule?

Controlling a project is key to the success or failure of the project. Earned Value Management (EVM) is a well-known technique to control the time and cost performance of a project and to predict the final project duration and cost. It is an easy tool to generate early warning signals to timely detect project problems or to exploit project opportunities. An overview of the EVM metrics is given in “Earned Value Management: An overview” and the formulas are summarized in “Earned Value Management: The EVM formulary”.

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.

?Figure 1: The PV, EV and AC S-curves

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 ≥ PVt and EV < PVt+1
ES = t + (EV - PVt) / (PVt+1 - PVt)

With
PVt The planned value at time t
EV:  The earned value at the current week

Example week 7:
EV at week 7 = € 90
t = week 5 since 85 ≤ 90 and 120 > 90
ES = 5 + (90 - 85) / (120 - 85) = 5.14

This value is illustrated graphically in figure 2.

?Figure 2: The Earned Schedule (ES) at week 7

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.