Modernising Project Forecasting with ISO Standards and Earned Schedule Modelling

Introduction

Project managers have long relied on Earned Value Management (EVM) to monitor performance and predict final outcomes. While EVM integrates cost, schedule, and scope data into a single system, its traditional methods often fall short when it comes to accurate early-stage forecasting. This article explores both the historical evolution of EVM and how the construction sector and others can adopt the international standards ISO 21508 and ISO 21511. It also presents cutting-edge research by Narbaev and De Marco, who developed a new CEAC (Cost Estimate at Completion) model using Earned Schedule and nonlinear regression via the Gompertz Growth Model.

Part I: A Brief History of EVM

EVM originated in the 1960s within the United States defence sector as a method to integrate cost and schedule performance. Key milestones include:

• PERT/Cost: Introduced in the early 1960s, this system attempted to forecast cost and schedule performance jointly but relied heavily on subjective judgement.

• C/SPEC and C/SCSC: In the late 1960s and 1970s, the U.S. Air Force introduced stricter guidelines requiring measurable progress and standardised reporting.

• ANSI/EIA-748 Standard: By the 1990s, EVM had become formalised through this standard, promoting consistency across industries.

• Earned Schedule (ES): Developed by Walt Lipke in 2003, ES added a time-based metric to traditional EVM, helping to overcome limitations in measuring schedule efficiency.

Today, EVM is used internationally and embedded within ISO standards, providing project professionals with a robust—albeit sometimes rigid—framework for performance tracking.

Part II: ISO 21508 and ISO 21511 – Establishing a Global Standard

The publication of ISO 21508:2018 (Earned Value Management in Project and Programme Management) and ISO 21511:2018 (Work Breakdown Structures for Project and Programme Management) marked a significant advancement in global project management standards. Developed collaboratively with input from IPMA, PMI, and experts from the UK, Australia, and other countries, these standards are based on established best practices, including the UK APM guidelines and Australian AS 4817.

ISO 21508 includes:

• Definitions and terminology

• Purpose and benefits of EVM

• Integration with wider project/programme management disciplines

• Process descriptions and EVM system requirements

• Application of EVM in cost and schedule performance analysis

It also features three annexes:

• Cost and schedule performance measurement

• Schedule analysis using Earned Schedule

• Integration with other project management processes

ISO 21511, meanwhile, standardises the development and application of Work Breakdown Structures (WBS), promoting consistency in scoping, scheduling, and budgeting activities.

How the Construction Sector Can Adopt ISO 21508 and 21511

The construction industry, with its high-risk profiles, numerous stakeholders, and tight margins, is particularly well-suited to benefit from these ISO standards. Adoption can be achieved through:

1. Training teams on EVM and WBS in line with ISO standards

2. Integrating ISO-aligned processes into existing project management systems

3. Using digital tools that support compliant EVM and WBS methodologies

4. Aligning contracts and reporting structures to reflect ISO requirements

5. Embedding consistent performance data into forecasting and reporting

6. Leveraging structured data for digital transformation initiatives, such as BIM and PMIS

   
   

By implementing these practices, construction firms can improve project visibility, control costs more effectively, and enhance decision-making.

Part III: Challenges with Traditional EVM Forecasting

Traditional CEAC forecasting, based on performance indexes such as the Cost Performance Index (CPI) and Schedule Performance Index (SPI), has two primary weaknesses:

• Retrospective Bias: It assumes historical performance trends will continue unchanged.

• Inaccuracy in Early Stages: Forecasts are often unreliable when projects are just beginning and data is limited or volatile.

These limitations reduce the predictive power of traditional EVM when early intervention is most critical.

Part IV: The Earned Schedule–Based Gompertz Model by Narbaev and De Marco

Narbaev and De Marco tackled these limitations by developing a new CEAC forecasting model using:

• Gompertz Growth Model (GGM): A nonlinear S-curve mimicking typical project cost patterns—slow start, rapid acceleration, and gradual completion.

• Nonlinear Regression: The model’s parameters (α, β, γ) are derived by fitting curves to actual and planned cost data.

• Earned Schedule Integration: Incorporates a Completion Factor (CF), derived from ES, to adjust cost forecasts based on schedule efficiency.

Two core forecasting equations emerge:

• Base Model: CEAC = AC + BAC × [GGM(1.0) - GGM(AT)]

• Schedule-Adjusted Model: CEAC = AC + BAC × [GGM(CF) - GGM(AT)]

Where:

• AC = Actual Cost

• BAC = Budget at Completion

• AT = Actual Time

• CF = Completion Factor

This approach enhances traditional EVM by allowing real-time forecasting using both cost and schedule dynamics.

Part V: Practical Advantages of the Model

The Gompertz-based CEAC model offers clear benefits:

• Improved Accuracy: Performs better than index-based methods across all project phases.

• Greater Precision: Demonstrates lower error margins and variance in tested projects.

• Ongoing Reliability: Remains robust as projects progress and more data becomes available.

• Scalability: Particularly beneficial for small to medium-sized projects where data is limited but precision is still critical.

Conclusion

The evolution of EVM—from its military origins to inclusion in ISO standards and advanced statistical modelling—demonstrates how project controls are adapting to increasing complexity. ISO 21508 and ISO 21511 provide a structured foundation, while the Gompertz-based CEAC model delivers sophisticated forecasting capabilities.

Together, these tools enable a proactive, data-informed approach to managing uncertainty and improving outcomes.

Call to Action

Project professionals, contractors, and PMOs should actively explore the integration of ISO 21508/21511 and the Earned Schedule–based Gompertz model into their forecasting practices. As projects become increasingly time-sensitive and resource-constrained, better foresight leads to better choices—and better results.

References:

International Organization for Standardization (ISO), 2018a. ISO 21508:2018 – Earned value management in project and programme management. Geneva: ISO.

International Organization for Standardization (ISO), 2018b. ISO 21511:2018 – Work breakdown structures for project and programme management. Geneva: ISO.

Narbaev, T. and De Marco, A., 2014. An earned schedule-based regression model to improve cost estimate at completion. Journal of Construction Engineering and Management, 140(2), pp.1–10. https://doi.org/10.1061/(ASCE)CO.1943-7862.0000797

Weaver, P., 2022. The origins and history of Earned Value Management. PM World Journal, [online] 11(8), pp.27–32. Available at: https://pmworldjournal.com/article/the-origins-and-history-of-earned-value-management [Accessed 1 May 2025].

Lipke, W., 2003. Schedule is different. The Measurable News, [online] March 2003, pp.31–34. Available at: https://earnedschedule.com/Docs/Schedule_Is_Different.pdf [Accessed 1 May 2025].

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