Background: Statements or commitments making use of numbers have an important place in government policy. They appear at all stages of the policy process: campaigning, formulation, monitoring and evaluation. Many types of sources are involved including research studies, national survey information, routine operational data collections and special systems devised to monitor particular initiatives. Method: The paper presents examples of policy uses of numerical evidence, and some sources of data that have been used to support them in the field of care for people with intellectual developmental disability in England. Conclusions: Different levels of precision or coverage are required at different stages of the process. Different types of numerical data are appropriate at the various stages of the policy process.