Does project performance stability exist? A re-examination of CPI and evaluation of SPI(t) stability

The development of the Earned Schedule (ES) method by Walt Lipke in 2003 has been shown to be an important extension to the Earned Value Management (HVM) method, increasing the utility of HVM data for project schedule analysis, control, and oversight. As ES provides a reliable time-based indicator of schedule performance, the objective of this article is to investigate whether the Schedule Performance Index (time) (SPI[t]) exhibited similar stability characteristics to those extensively reported for the Cost Performance Index (CPI)¹ in H VM. This article analyses fcHi data from three different countries for projects in three industry segments. There were 37 projects examined for SPI(t) stabilit)' and 26 for CPI stability. It has been found that while the behavior of SPI (t) is broadly consistent with CPI, the widely reported CPI stability rule cannot be generalised even within the U.S. Department of Defense (DoD) project portfolio. Further research is required to develop improved understanding of prefect performance characteristics and the behavior of CPI and the SPI(t).