While HM Treasury is an important user of department’s models, it does not comprehensively scrutinise and challenge a department’s quality assurance arrangements for the models it uses. There is no requirement for the Aqua Book to be followed as part of the investment approval process nor as part of the spending review process.29 HM Treasury told us it would want assurance that the department had taken the Aqua Book and its guidance into account, but it does not specifically check.30