Nothing to See Here? A non-inferiority approach to parallel trends

Understand

Difference-in-differences is a popular method for observational health policy evaluation.

• It relies on a causal assumption that in the absence of intervention, treatment groups' outcomes would have evolved in parallel to those of comparison groups.
• Researchers frequently look for parallel trends in the pre-intervention period to bolster confidence in this assumption.
• The popular "parallel trends test" evaluates a null hypothesis of parallel trends and, failing to find evidence against the null, concludes that the assumption holds.