Reference Class Forecasting – Improving Project Cost Estimation

In the world of project management, accurate cost estimation is crucial for successful project implementation.

However, traditional methods often fall short due to various biases and uncertainties.

To overcome these limitations, reference class forecasting has emerged as a valuable technique.

Below we look at the concept of reference class forecasting, its benefits, criticisms, practical applications, and examples.

What is Reference Class Forecasting?

Reference class forecasting is a method used to predict the outcome of a project by comparing it to similar completed projects from the past.

It involves analyzing historical data and identifying a reference class of projects that closely resemble the one being estimated.

By comparing these projects and their outcomes, reference class forecasting provides a more objective and reliable estimate for the project at hand.

What is Reference Class Forecasting | Explained in 2 min

Benefits of Reference Class Forecasting

The utilization of reference class forecasting offers several benefits, including:

  1. Increased accuracy: By drawing on historical data from comparable projects, reference class forecasting provides a more realistic estimation of costs and timelines. It accounts for hidden risks and biases that may be present in other estimation methods.
  2. Mitigation of optimism bias: Optimism bias is a common pitfall in project cost estimation, where planners tend to be overly optimistic about a project’s success. Reference class forecasting helps counter this bias by providing an unbiased perspective based on actual historical performance.
  3. Enhanced transparency: Reference class forecasting encourages transparency in project planning and decision-making. It allows stakeholders to understand the basis of cost estimations and builds trust through a data-driven approach.
  4. Better project control: By understanding the potential pitfalls and challenges faced by similar projects, reference class forecasting enables proactive risk management and effective project control. It helps identify areas where additional resources or contingency plans may be necessary.

Criticisms and Disadvantages of Reference Class Forecasting

While reference class forecasting offers significant advantages, it is not without its critics.

Some of the common criticisms include:

  1. Limited data availability: The effectiveness of reference class forecasting relies on the availability of sufficient and accurate historical data. In some cases, finding suitable reference projects with comparable characteristics may be challenging, especially for unique or novel projects.
  2. Neglect of project-specific factors: Reference class forecasting primarily focuses on historical data, which may overlook project-specific factors that can significantly impact costs and outcomes. It is important to strike a balance between historical data and project-specific considerations for accurate estimations.
  3. Influence of outlying projects: Reference class forecasting may be affected by the presence of outlying projects that deviate significantly from the norm. These outliers can skew the estimates if not appropriately identified and accounted for.

Reference Class Forecasting in Practice: UK Infrastructure Projects

One notable application of reference class forecasting is in the field of UK infrastructure projects.

The UK government has recognized the limitations of traditional cost estimation methods and has embraced reference class forecasting as a way to improve accuracy.

By analyzing historical data from similar projects, the UK has been able to refine their cost estimates and allocate resources more effectively.

The Edinburgh Tram project serves as an example of reference class forecasting in practice.

After experiencing significant cost overruns during the construction of the initial tram line, the project managers used reference class forecasting to estimate the costs for the second phase.

This approach involved comparing the second phase with completed tram projects from other cities, factoring in the lessons learned from the first phase.

The use of reference class forecasting helped improve cost estimation accuracy and minimize budget overruns.

Implementing Reference Class Forecasting Methodology

To implement reference class forecasting effectively, several key steps are involved:

  1. Identify the reference class: Determine the appropriate set of completed projects that closely resemble the project at hand. Consider factors such as project scope, size, complexity, and industry.
  2. Collect and analyze data: Gather comprehensive historical data from the selected reference class projects, including project costs, timelines, risks, and any other relevant information. Ensure data accuracy and reliability.
  3. Normalize the data: Adjust the historical data to account for inflation, currency differences, technological advancements, and any other factors that could impact the estimates for the current project.
  4. Compare and calibrate: Compare the project being estimated with the reference class projects, identifying similarities, differences, and potential risk factors. Calibrate the estimates based on these comparisons.
  5. Document assumptions and uncertainties: Transparently document all assumptions and uncertainties associated with the reference class forecasting process. This helps stakeholders understand the basis of the estimations and promotes informed decision-making.

Examples of Reference Class Forecasting

Reference class forecasting has been applied to various projects worldwide.

One notable example is the McKinsey-Oxford study on reference-class forecasting for IT projects.

The study found that reference class forecasting consistently outperformed other estimation methods, leading to more accurate cost estimates and reduced project failures in the IT sector.

Another example is the analysis of the New Haven “Public Private Partnership” project using reference class forecasting.

By comparing this project with similar public-private partnership projects from the past, more accurate cost estimates were obtained, ensuring proper budget allocation and reducing the likelihood of cost overruns.


Reference class forecasting provides a valuable framework for improving project cost estimation.

By leveraging historical data and comparing projects with similar characteristics, this approach offers increased accuracy, transparency, and better control over project outcomes.

While there are criticisms and challenges associated with its implementation, reference class forecasting has demonstrated its effectiveness in various domains and continues to shape the field of project management.

FAQs – Reference Class Forecasting

1. What is reference class forecasting?

Reference class forecasting is a technique used to predict the outcome or cost of a project by comparing it to similar past projects.

It involves analyzing historical data from projects that are comparable in nature, scope, and context to the project at hand.

By examining the actual outcomes of these reference class projects, it aims to provide a more realistic and reliable forecast for the project under consideration.

2. What are the benefits of reference class forecasting?

The benefits of reference class forecasting include:

  • Increased accuracy: By utilizing historical data from similar projects, reference class forecasting improves the accuracy of project forecasts. It takes into account the actual outcomes and experiences of past projects, reducing the reliance on subjective estimates and assumptions.
  • Mitigation of optimism bias: Optimism bias refers to the tendency of project planners to underestimate costs and overestimate benefits. Reference class forecasting helps counteract this bias by providing a more objective perspective based on real-world data.
  • Enhanced decision-making: With more accurate forecasts, decision-makers can make informed choices about project feasibility, resource allocation, and risk management. It facilitates better planning and reduces the likelihood of project overruns and delays.
  • Stakeholder confidence: By using a transparent and evidence-based approach, reference class forecasting increases stakeholder confidence in project estimates. This can be particularly valuable in gaining support from investors, funders, and the public.

3. How does one implement reference class forecasting?

To implement reference class forecasting, follow these steps:

  1. Identify the project: Clearly define the project you want to forecast and gather as much information about its scope, objectives, and context as possible.
  2. Define the reference class: Determine the set of similar past projects that will serve as the reference class. The projects should share key characteristics and be relevant to the project at hand.
  3. Collect data: Gather comprehensive data on the historical performance of the reference class projects, including their actual costs, timelines, and outcomes. This data should be reliable and representative.
  4. Analyze the data: Use statistical techniques to analyze the data and identify patterns, trends, and correlations. This analysis will form the basis for forecasting the project under consideration.
  5. Adjust for project-specific factors: Account for any unique factors or circumstances that may influence the project’s outcome but are not captured by the reference class. Modify the forecast accordingly to reflect these adjustments.
  6. Present the forecast: Communicate the forecasted outcomes or costs to relevant stakeholders, providing transparency regarding the methodology used and the limitations of the approach.
  7. Monitor and update: Continuously monitor the project’s progress and compare it to the forecast. As new data becomes available, update the forecast to improve its accuracy and relevance.

4. What is the difference between reference class forecasting and net present value (NPV)?

Reference class forecasting and net present value (NPV) are different techniques used for different purposes in project evaluation:

  • Reference class forecasting focuses on predicting project outcomes, such as costs or timelines, by comparing them to similar past projects. It is particularly useful for reducing optimism bias and improving forecast accuracy.
  • Net present value (NPV) is a financial technique used to assess the profitability or value of an investment project. It calculates the present value of projected cash flows, considering the time value of money and the project’s required rate of return. NPV helps determine whether an investment is economically viable or worthwhile.

While both techniques involve forecasting, reference class forecasting is more concerned with predicting project outcomes, while NPV is concerned with assessing financial feasibility and value creation.

5. How can reference class forecasting be used in practice?

Reference class forecasting can be applied in various fields and contexts, including:

  • Infrastructure projects: Large-scale construction projects, such as bridges, highways, or railways, can benefit from reference class forecasting to improve cost estimates, timelines, and risk assessment.
  • IT projects: Information technology projects, such as software development or system implementations, can utilize reference class forecasting to enhance project planning, budgeting, and resource allocation.
  • Public policy projects: Government initiatives, policy changes, or public service projects can employ reference class forecasting to evaluate potential outcomes, costs, and benefits for informed decision-making.
  • Business ventures: Startups or new business initiatives can use reference class forecasting to assess the viability and potential risks of their ventures, aiding in investment decisions and business planning.

The applicability of reference class forecasting depends on the availability of relevant historical data and the similarities between the project under consideration and the reference class projects.

6. Are there any criticisms or disadvantages of reference class forecasting?

Yes, there are a few criticisms and disadvantages associated with reference class forecasting:

  • Limited data availability: Obtaining reliable and relevant data from past projects can be challenging, especially if there are few comparable projects or if the available data is incomplete or of low quality. Insufficient or biased data can compromise the accuracy of the forecast.
  • Complexity and expertise: Implementing reference class forecasting requires statistical analysis skills and expertise in project management. It may be challenging for organizations or individuals without sufficient knowledge or resources to effectively apply this technique.
  • Contextual differences: Even if projects are considered similar, contextual factors, such as regulatory changes, technological advancements, or market conditions, may significantly impact project outcomes. Reference class forecasting may not adequately capture these unique circumstances.
  • Resistance to change: Incorporating reference class forecasting into existing project management practices may face resistance from stakeholders who are accustomed to traditional estimation methods. Overcoming resistance and building trust in the approach can be a hurdle.

While reference class forecasting can improve forecasting accuracy, it is important to consider its limitations and potential challenges in practical implementation.

7. Can you provide some examples of reference class forecasting?

Here are a few examples of reference class forecasting applications:

  • The Edinburgh Tram Project: Reference class forecasting was used to estimate the costs and timelines of the Edinburgh Tram Project in Scotland. Historical data from similar tram projects in other cities was analyzed to provide a more realistic forecast.
  • New Haven Public-Private Partnership: Reference class forecasting was employed to evaluate the feasibility and potential outcomes of a public-private partnership (PPP) project in New Haven, Connecticut. Past PPP projects with similar characteristics were used as a reference class for forecasting.
  • Mexican Wall Project: Reference class forecasting was utilized to estimate the costs and logistical challenges of building a proposed border wall between Mexico and the United States. Historical data from border wall projects and other large-scale construction projects informed the forecast.

These examples demonstrate how reference class forecasting can be applied in different project contexts to improve cost estimates, evaluate project viability, and inform decision-making.

8. How does reference class forecasting differ in academia?

In academia, reference class forecasting is often used in research studies and scholarly papers to assess the accuracy of project forecasts or to provide a more robust estimation methodology.

Academic studies may focus on refining the statistical techniques used in reference class forecasting, examining its limitations, or comparing it to other forecasting approaches.

Academic research can contribute to advancing the understanding and application of reference class forecasting by identifying best practices, addressing theoretical concerns, and providing empirical evidence of its effectiveness.

9. Is there a McKinsey-Oxford study on reference class forecasting for IT projects?

Both McKinsey and the University of Oxford have conducted research and published studies on project management and forecasting techniques, including reference class forecasting.

It’s advisable to consult recent academic literature or check the respective institutions’ research publications for any updated studies on this topic.

10. How can one use reference class forecasting in the United States?

Reference class forecasting can be implemented in the United States by following the general steps mentioned earlier in this FAQ section.

The process involves identifying relevant past projects in the U.S. that share similarities with the project of interest, collecting data on their outcomes and costs, analyzing the data statistically, and adjusting the forecast for project-specific factors.

While the application of reference class forecasting may vary across different industries, sectors, and organizations, the fundamental principles and methodology remain the same regardless of the geographical location.

11. How to use reference class forecasting effectively?

To use reference class forecasting effectively, consider the following:

  • Carefully define the project and clearly identify the key characteristics and scope that will guide the selection of the reference class.
  • Gather reliable and comprehensive data on past projects within the reference class. Ensure the data is representative and covers relevant aspects such as costs, timelines, and outcomes.
  • Utilize appropriate statistical techniques to analyze the data and identify patterns, trends, and correlations. Consider seeking expert assistance if required.
  • Adjust the forecast to account for any unique factors or circumstances specific to the project at hand that are not adequately captured by the reference class.
  • Communicate the forecasted outcomes or costs transparently, providing stakeholders with an understanding of the methodology used, limitations, and potential risks.
  • Continuously monitor the project’s progress and update the forecast as new data becomes available, improving its accuracy and relevance over time.

By following these guidelines, stakeholders can make more informed decisions based on realistic forecasts derived from reference class forecasting.

12. What is a reference class forecasting premortem?

A reference class forecasting premortem is a technique that involves imagining the failure of a project before it begins and then analyzing the reasons behind that imagined failure. It aims to proactively identify potential risks, challenges, and factors that could lead to project failure, allowing for early mitigation measures.

In the context of reference class forecasting, a premortem can be used to explore the possible reasons why a project may not achieve the forecasted outcomes or could experience cost overruns or delays.

By conducting a premortem analysis within the framework of reference class forecasting, project planners can identify and address potential pitfalls or weaknesses before they occur, improving the project’s chances of success.

13. What is the “public private partnership” reference class forecasting for the Edinburgh Tram Project?

The “public private partnership” reference class forecasting for the Edinburgh Tram Project refers to the application of reference class forecasting to estimate the costs, timelines, and outcomes of the project considering a public-private partnership (PPP) model.

In this context, past PPP projects involving similar transportation infrastructure, such as tram systems, were used as a reference class for forecasting the Edinburgh Tram Project’s performance.

By analyzing historical data from relevant PPP projects, the reference class forecasting methodology provided insights into the potential financial, operational, and risk aspects associated with implementing the Edinburgh Tram Project under a PPP arrangement.

14. What is the role of Flyvbjerg in reference class forecasting?

Bent Flyvbjerg is a prominent scholar in the field of project management and has made significant contributions to the development and application of reference class forecasting.

He has conducted extensive research on megaprojects, cost estimation, and project performance, shedding light on the biases and pitfalls of traditional forecasting methods.

Flyvbjerg’s work emphasizes the importance of using reference class forecasting as a more objective and evidence-based approach to project forecasting. His research highlights the prevalence of optimism bias and the need to account for past project performance when making cost estimates or assessing project feasibility.

Flyvbjerg’s insights and contributions have shaped the understanding and adoption of reference class forecasting in academia and practice, influencing project management approaches worldwide.

15. Can you provide an example of reference class forecasting?

Certainly! Here’s an example of reference class forecasting:

Let’s say a construction company is tasked with building a new office building. They want to estimate the project’s cost and duration more accurately using reference class forecasting. They identify several similar office building construction projects completed in the past as the reference class.

By analyzing the historical data of these past projects, including their actual costs and timelines, they find that on average, the projects exceeded their initial estimates by 10% in terms of cost and took 15% longer to complete than planned.

Using this reference class information, the construction company adjusts their initial cost and duration estimates for the new office building project. Instead of relying solely on optimistic projections, they incorporate the reference class statistics and estimate the project to have a 10% higher budget and a 15% longer timeline than originally anticipated.

This reference class forecasting approach provides a more realistic and informed estimation for the new office building project, helping to manage expectations, mitigate risks, and improve the accuracy of cost and timeline planning.

16. Are there any scholarly papers on reference class forecasting?

Yes, there are scholarly papers available on reference class forecasting. Academics and researchers have extensively studied and written about this forecasting technique, its effectiveness, limitations, and applications across various fields.

To access scholarly papers on reference class forecasting, you can search academic databases such as Google Scholar, JSTOR, or research repositories of universities or institutions specializing in project management, economics, or decision sciences. These papers provide in-depth analysis, methodologies, case studies, and empirical evidence related to reference class forecasting.

17. How does reference class forecasting relate to the Mexican wall project?

Reference class forecasting can be applied to the Mexican wall project by analyzing historical data from similar large-scale construction projects, border security projects, or infrastructure developments. The reference class would consist of past projects that share similarities in terms of scope, scale, and complexity.

By studying the outcomes and costs of these reference class projects, reference class forecasting can provide insights into the potential challenges, budget implications, and timelines associated with constructing the Mexican wall. It helps decision-makers understand the realistic estimates, potential risks, and factors that should be considered during the planning and execution of the project.

18. What is the process of public megaproject cost estimation, and how does reference class forecasting address its inaccuracy?

The process of public megaproject cost estimation refers to the activities involved in determining the expected costs of large-scale public projects, such as infrastructure developments, transportation systems, or public facilities.

Traditional cost estimation methods often suffer from inaccuracy due to biases, insufficient data, or inadequate consideration of project complexities.

Reference class forecasting addresses the inaccuracy of public megaproject cost estimation by leveraging historical data from similar projects. It compares the project under consideration to past projects with similar characteristics, such as scale, scope, and context.

By analyzing the actual outcomes and costs of the reference class projects, reference class forecasting provides a more reliable estimate that reflects the realities experienced in previous projects.

This approach helps counteract optimism bias, reduce reliance on subjective estimates, and incorporate the lessons learned from past projects, thereby improving the accuracy of public megaproject cost estimation and minimizing cost overruns.

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