Can understanding mechanisms solve the problem of extrapolating from study to target populations (the problem of 'external validity')?
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We all hope that study results will benefit target populations. However, experience suggests that extrapolation based on simple induction can be problematic. Here we have argued that examining mechanisms of action in study and target populations can help justify successful extrapolation and also guide adequate descriptions of interventions. Our proposed method involves (i) establishing that common problems with obtaining knowledge of mechanisms of action have been overcome; (ii) verifying that relevant mechanisms in the study and target populations are similar; and (iii) establishing that study and target contexts are relevantly similar.
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