Evidence, external validity and explanatory relevance.

When does one fact speak for another? That is the problem of evidential relevance. Peter Achinstein’s answer, in brief: Evidential relevance = explanatory relevance.2 My own recent work investigates evidence for effectiveness predictions, which are at the core of the currently heavily mandated evidencebased policy and practice (EBPP): predictions of the form ‘Policy treatment T implemented as, when and how it would be implemented by us will result in targeted outcome O.’ RCTs, or randomized controlled trials, for T and O are taken to be the gold standard for evidence for effectiveness predictions. I question this: Not just whether they are gold-standard evidence, but more, How can they be evidence at all? What makes them relevant to the truth of the prediction that T will work for us