Nothing to See Here? A Non‐Inferiority Approach to Parallel Trends

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. This tightly controls the probability of falsely concluding that trends are not parallel, but may have low power to detect non-parallel trends. When used as a screening step, it can…

• AF
  Agency for Healthcare Research and Quality

  Award: R01HS028985, LH
• NI
  National Institute of General Medical Sciences

  Award: 1R35GM155224, AB