You can't do a study that way. People do or do not wear masks based on emergent social acceptance, regardless of government actions. If you want to do a discontinuity study you have to evaluate actual mask usage in the real world and discover a boundary discontinuity. You can't just do it by jurisdiction and assume that the government proclamations work. People wear masks when they aren't required, and they may not wear them when they are required.
Yet we’ve been hearing since the beginning of 2021 that they’ve been proven to have been much more efficient by the coalition parliament which has come up with this idea.