The plausibility of the ``parallel trends assumption'' in Difference-in-Differences estimation is usually assessed by a test of the null hypothesis that the difference between the average outcomes of both groups is constant over time before the treatment. However, failure to reject the null hypothesis does not imply the absence of differences in time trends between both groups. We provide equivalence tests that allow researchers to find evidence in favor of the parallel trends assumption and thus increase the credibility of their treatment effect estimates. While we motivate our tests in the standard two-way fixed effects model, we discuss simple extensions to settings in which treatment adoption is staggered over time.

翻译：在双重差分估计中，“平行趋势假设”的合理性通常通过对原假设进行检验来评估，该原假设认为处理前两组平均结果的差异随时间保持不变。然而，未能拒绝原假设并不表明两组时间趋势不存在差异。我们提供了等价性检验，使研究者能够找到支持平行趋势假设的证据，从而提高其处理效应估计的可信度。尽管我们在标准双向固定效应模型中论证了这些检验的动机，但我们还讨论了将其简单扩展至处理采用随时间交错进行的情形。