In a difference‐in‐difference framework, the parallel trends assumption means that
A. The treatment and control groups look similar in the pre‐treatment period.
B. The treatment and control groups look similar in the post‐treatment period.
C. The treatment and control groups look similar in both periods.
D. The difference between the treatment and control groups in the post‐period would have been the same as in the pre‐period if not for the treatment.
Answer: D. The difference between the treatment and control groups in the post-period would have been the same as in the pre-period if not for the treatment.