What is Optimism Bias?
Research from the UK Treasury and other international bodies has conclusively shown that project cost estimates are typically optimistic, with actual costs exceeding initial estimates by significant margins. This is not due to poor estimating practice or deliberate manipulation, but rather an inherent human cognitive bias towards optimism when planning future activities.
Common manifestations of optimism bias in cost estimating include:
- Underestimating complexity: Assuming construction will proceed smoothly without delays or difficulties
- Overlooking risks: Failing to adequately account for known risks or identify emerging risks
- Base rate neglect: Ignoring historical performance data in favor of project-specific optimism
- Planning fallacy: Focusing on best-case scenarios rather than typical outcomes
- Scope creep blindness: Not anticipating scope growth that typically occurs during design development
- Strategic misrepresentation: Consciously or unconsciously presenting low estimates to secure approval
PCEM Measures to Counteract Optimism Bias
The PCEM framework incorporates several specific measures designed to counteract optimism bias and produce more realistic estimates:
1. Mandatory Historical Data Reference
Step 6 requires estimators to reference Queensland's SmartCost database of completed project costs. This grounds estimates in actual historical performance rather than optimistic projections. Significant deviations from historical benchmarks must be explicitly justified.
2. Structured Risk Assessment
Step 9 mandates systematic risk identification and quantification. This forces explicit consideration of what could go wrong, counteracting the natural tendency to assume everything will proceed according to plan. Risk workshops involving diverse stakeholders help identify risks that individual estimators might overlook.
3. P50/P90 Confidence Levels
Business case estimates must present both P50 (median, 50% confidence) and P90 (90% confidence) values. This explicitly acknowledges uncertainty and ensures governance decisions are made with realistic expectations rather than optimistic base estimates.
4. Independent Peer Review
Step 13 requires independent validation of all estimates. Fresh eyes from qualified estimators not invested in the project outcome provide objective assessment and challenge optimistic assumptions. Concurrence review for projects over $25 million provides an additional layer of independent scrutiny.
5. Reality Checks and Benchmarking
Step 8 mandates comparison against independent benchmarks and alternative calculation methods. This catches estimates that are significantly optimistic compared to industry norms or similar completed projects.
6. Documented Assumptions and Exclusions
Throughout the process, estimators must document all assumptions, exclusions, and areas of uncertainty. This transparency makes optimistic assumptions visible and challengeable, rather than buried in the estimate.
Practical Application
Effective estimators actively question their own optimism throughout the process:
- When developing quantities, ask "What have I missed?" rather than assuming completeness
- When selecting rates, favor actual historical data over theoretical calculations
- When assessing duration, reference actual completed projects rather than ideal scenarios
- When identifying risks, challenge the team to think of unlikely but possible events
- When setting contingency, resist pressure to minimize in favor of realistic uncertainty allowances
"The greatest enemy of a good cost estimate is the optimism of the estimator. The PCEM process is deliberately designed to counteract this bias through structured analysis, historical grounding, risk quantification, and independent review. Following the process rigorously produces estimates that are credible, defensible, and realistic."