Knowing what we are talking about: why evidence doesn't always travel

When is a well-established study result that a given policy/programme/treatment produced a given outcome in a particular study setting ('there') evidence that that policy/programme/treatment will produce that outcome in a new setting ('here')? This paper insists that 'there' and 'here' be firmly distinguished and offers in answer that we must have evidence that two further facts obtain: (a) that the policy can play the same causal role widely (widely enough to cover both here and there) and (b) that a complete set of the support factors necessary for the policy to operate here are present in some individuals here.