The proposition that public policy should be ‘evidence-based’ is now widely accepted (although there is still considerable contestation around the meaning, nature, types, and qualities of evidence, the interpretation of evidence, the politics of evidence etc). The evidence in the phrase ‘evidence-based policy’ is often portrayed as evidence about ‘what works, where and for whom’: the ‘what’ might be a policy or technical intervention, and the ‘works’ is understood as the ability to deliver a particular outcome.
There is a second area of evidence that is less commonly referred to in debates about evidence-based policy. This is evidence about ‘what is, what has been and what is likely to be’, which provides a picture (or more likely multiple, partial and contested pictures) of key structures, institutions, alliances, power relations, drivers, trends, outcomes, dynamics and pathways within a particular sector or policy area. Here there is a critical role for historical evidence as it allows for some understanding of what might be thought of as the ‘baseline of change’.