Evaluation of the NHS England evidence-based interventions programme: a difference-in-difference analysis.

Moreover, one review of DiD studies found that pre-trend testing is rarely used in studies with large numbers of observations as even small differences in pre-implementation trends are likely to be significant. 18 The authors of this review highlight how pre-trend testing is more often used in studies with small numbers of observations when the test could fail to reject parallel trends because it is underpowered rather than due to parallel trends. 18 For the second assumption, we constructed a control group of low value procedures that should not have been impacted by disinvestment in the low value procedures targeted by the EBI programme during our period of analysis.

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