Applying Statistical Forecasting of Project Duration To Earned Schedule-Longest Path


Walt Lipke

PMIÒ Oklahoma City Chapter

Oklahoma, USA


Project duration forecasting has been enhanced with the introduction and application of the techniques derived from Earned Schedule (ES). The computed forecast results from ES have been shown to be better than any other Earned Value Management based method using both real and simulated performance data. Even so, research has shown that as the topology of the network schedule becomes more parallel, the accuracy of the ES forecast worsens. Recently, forecast accuracy improvement has been achieved for highly parallel type schedules with the method of Earned Schedule-Longest Path. This paper proposes further advancement to the longest path approach through anomaly rejection and the application of statistical methods.


To assist the reader’s understanding beyond his/her knowledge of Earned Value Management (EVM), three areas will necessarily be discussed: Earned Schedule (ES), Longest Path (LP), and Statistical Forecasting. Some may not need the review. However, for those readers not well-versed in the publications on these topics, the introduction, summarizing several papers, should be helpful [Lipke, 2003; Lipke, 2010; Lipke, 2012-2].

Earned Schedule. The concept is illustrated in figure 1. ES is the measure of time indicating the completed portion of Planned Duration from the EVM performance measurement baseline (PMB). The measure facilitates the ability to assess the schedule performance efficiency; i.e., the time-based schedule performance index, SPI(t). The index is equal to ES divided by AT, the actual time duration from the project start to the status point.

The derived schedule efficiency, SPI(t), in turn, enables the forecasting of project duration through the simple formula [Henderson, 2004]:

IEAC(t) = PD / SPI(t)

where             IEAC(t) = Independent Estimate at Completion (time units)

PD = Planned Duration


Figure 1. Earned Schedule Concept

The forecasting capability of the formula has been shown to be reasonably good. It has been verified by simulation [Vandevoorde and Vanhoucke, 2007] and application to real project data [Henderson, 2003]. As well, the capability has been verified elsewhere, thereby establishing a compelling argument for applying ES forecasting when EVM is employed for project control [Lipke, 2009; Lipke, 2014]. However, recent research has demonstrated that the topology of the schedule has impact on the “goodness” of the forecast. The ES forecasts are more accurate for schedules which are more serial and less so when parallel [Vanhoucke, 2009]. 


To read entire paper (click here)

About the Author

Walt-LipkeWalt Lipkeflag-usa

Oklahoma, USA 

Walt Lipke retired in 2005 as deputy chief of the Software Division at Tinker Air Force Base in the United States. He has over 35 years of experience in the development, maintenance, and management of software for automated testing of avionics. During his tenure, the division achieved several software process improvement milestones, including the coveted SEI/IEEE award for Software Process Achievement. Mr. Lipke has published several articles and presented at conferences, internationally, on the benefits of software process improvement and the application of earned value management and statistical methods to software projects. He is the creator of the technique Earned Schedule, which extracts schedule information from earned value data.

Mr. Lipke is a graduate of the USA DoD course for Program Managers. He is a professional engineer with a master’s degree in physics, and is a member of the physics honor society, Sigma Pi Sigma (SPS). Lipke achieved distinguished academic honors with the selection to Phi Kappa Phi (FKF). During 2007 Mr. Lipke received the PMI Metrics Specific Interest Group Scholar Award. Also in 2007, he received the PMI Eric Jenett Award for Project Management Excellence. The award honored his leadership role and contribution to project management resulting from his creation of the Earned Schedule method. At the 2013 EVM Europe Conference, he received an award in recognition of the creation of Earned Schedule and its influence on project management, EVM, as well as schedule performance research. Most recently, the College of Performance Management awarded Mr. Lipke the Driessnack Distinguished Service Award, their highest honor.

Walt Lipke can be contacted at [email protected]