Earned Schedule (ES) is emerging as a valuable tool in the overall management of projects. In our view, the ES methodology provides a valuable adjunct to, and a useful sanity check of, the ‘critical path’ (CPM) schedule developed for a project, for very little effort – provided the project has implemented Earned Value Management (EVM).
Using the same data as EVM, ES ‘scales’ the remaining duration of the project based on the volume of work accomplished to date compared to the planned volume, as measured by SPI(t). SPI(t) has the potential to predict future slippages that the CPM schedule may not be indicating.
CPM schedules have two major flaws inherent in the methodology:
a. CPM assumes all future work will be accomplished as planned. There is no ‘scaling function’ for the performance of future work similar to the EVM calculations of EAC = BAC/CPI for cost.
b. CPM schedules do not show critical path slippage if ‘float’ is being consumed. The ‘bow wave’ of delayed work eventually becomes critical (usually with disastrous consequences) but there may not be pre-warning. (for more on scheduling see: http://www.mosaicprojects.com.au/Planning.html)
Research by Henderson and Zwikael has demonstrated that CPI and SPI(t) are closely correlated at a summary level across a range of commercial projects. This and other research suggests ES will provide a valuable management insight to help the successful delivery of projects, but whilst this debate needs to be finalised it will ultimately be determined in the marketplace.
The Earned Schedule tools are freely available from http://www.earnedschedule.com together with published papers and links to other sites.