Update Optimism Bias Guidance
Edinburgh Tram Inquiry · Edinburgh Tram Inquiry Report · Issued 1 August 2023 · Addressed to: Scottish Government
Source — verbatim from the inquiry
●Inquiry recommendation, Chapter 25 Recommendation 8 (paragraph 25.220)
The existing Guidance on optimism bias was based on empirical data available almost two decades ago and should be revised to take into account the additional data that is now available. In particular, the reference classes should be updated to include a specific reference to light rail projects and the recommended uplifts for the different reference classes should be adjusted to reflect the additional empirical data that is now available. Thereafter the Guidance should be reviewed and revised to take account of additional data on a regular basis at intervals of not more than five years.
Edinburgh Tram Inquiry, Edinburgh Tram Inquiry Report · 1 Aug 2023 Source PDF →
Published evidence summary
Publicly available evidence relating to this recommendation:
- No published update to optimism bias guidance incorporating light rail project data or reflecting current empirical evidence has been identified to March 2026.
Sources
How was this evidence gathered?
Response — verbatim from government
●Scottish Government
The Scottish Government stated it already operates in line with best practices but will consider updating guidance. Source: Transport Secretary Statement, 2 November 2023.
Scottish Government · 2 Nov 2023 Written response →
Evidence trail — what's actually happened since
- 2 Nov 2023 Initial status based on Scottish Government and City of Edinburgh Council responses to the Edinburgh Tram Inquiry Report (September 2023). Source →
Each entry above links to a primary source — gov.uk written statement, consultation response document, or inspection report. The Index does not characterise government intent; it tracks what has been published.
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Source and Response are verbatim from primary documents. The Evidence trail records published activity since — written statements, consultation outcomes, inspection findings, parliamentary references. The Index does not paraphrase or characterise intent; it tracks what has been published. Where the evidence is the absence of action (a missed deadline, a slipped timetable), that absence is documented from primary sources rather than inferred.
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