HM Treasury has published an update to the government’s central guidance on evaluation that is described as the “most significant revision” in standards since 2020.
The Treasury’s so-called Magenta Book is a critical tool supporting the effective development and assessment of government policies, programmes and projects.
It is written for the policy, delivery and analytical professions with the aim of ensuring that good evidence is built into the design and delivery of projects from the earliest stages to maximise understanding of impacts.
The document aligns with HM Treasury’s Green Book guidance on how to assess and evaluate the costs, benefits, and risks of policies, programmes, and projects.
The joint Cabinet Office and HM Treasury Evaluation Task Force-led work on the latest edition of the Magenta Book, which was published on Friday and runs to 160 pages.
It said the revised guidance “marks a major milestone for evidence-based policymaking” and describes the changes as “moving the needle from ‘measuring what happened’ to ‘learning how to improve in real-time'”.
The revised Magenta Book introduces several new or significantly expanded areas of focus to support high-quality evaluation across government. They include a redrafted section on value for money – and a new annex – providing a clearer definition and expanded methodologies for assessing the best use of public resources, including “non-monetisable” social costs and benefits.
Elsewhere in the updated Magenta Book, new guidance addresses the responsible and ethical use of artificial intelligence in social research and evaluation. New principles for research transparency also feature, including recommendations for pre-registering evaluation protocols and using open-source platforms like GitHub for sharing code.
For the first time, a new section on place-based evaluation offers specific considerations for evaluating interventions designed for particular geographic areas.
And there is also a dedicated new annex that frames “test and learn” as a way of working that uses social research to iteratively adapt and optimise interventions before full-scale impact assessment.
Head of government social research Jenny Dibden, finance function head Conrad Smewing, and policy profession head Susan Acland-Hood worked with the Evaluation Task Force on the Magenta Book update.
In the foreword to the 2026 edition, they say the principles in the updated guidance will “empower” social research, finance and policy professionals to deliver better evidence, better decisions and ultimately better outcomes for the public.
“Proportionate and high-quality evaluation is a strategic necessity that ensures accountability, informs future spending decisions, and drives the iterative improvement of public services,” they wrote.
“The strength of this updated version of the Magenta Book guidance lies in its integrated perspective. In addition to providing refreshed technical guidance for analytical specialists, new guidance on value for money evaluation, test and learn approaches to policy development, and the alignment of benefits management with monitoring and evaluation ensures the new Magenta Book can serve as a shared roadmap for evaluation across the policy, delivery, finance, and analytical professions.”