Normally, the anticipation effect is tested as an assumption for Diff-in-Diff. However, it seems that it is necessary for examining the impact of laws on firms' behavior or else. For example, we want to test if the anti-collusion affects firms' assets growth. Therefore, we expect that firms will know about the establishment of the laws(due to media, government discussion, insider information), leading to firms change their assets growth before the actual event dates.

On the other hand, I am wondering if we need to test anticipation effect if the event date is a natural event ( not a laws, etc), saying an example that the level of vaccinated people/population is 30% or a tsunami, I do not think anticipation test is needed, can I ask your opinion then?