Fig. 3: Parallel trend checks.

The parallel trend test needs to satisfy the principle of ex ante and ex post significance. The confidence intervals of the coefficients of the pre- period in the figure are all through 0, indicating that there is no significant difference between the cities before the implementation of the policy; the coefficients of the post period have no significant difference at the beginning, but in the 5th and 6th period, the confidence intervals of the coefficients do not go through 0, which indicates that the implementation of the policy resulted in a significant difference, and indicates that the test for parallel trends was passed.