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thethe i want to thank you for this timely and beautiful graph
if you look at it you will note that cases in North Dakota were running above cases in South Dakota when the governor of North Dakota announced the mask mandate on November 14...and when it lapsed on January 18, the number of cases in North Dakota were below those in South Dakota
I think the cases data can be problematic because access to testing might have differed in the two states...changes in cases may reflect changes in testing access
So I would focus on deaths, where measurement issues are less likely to be a problem.
With deaths there is a lag of a couple weeks before a change in policy can be expected to affect the data. So I would look at deaths in the December and January following the implementation of the mask mandate in North Dakota.
At the end of November 2020, deaths from covid were 954 in North Dakota and 946 in South Dakota. About the same. Over the next two months (December and January) deaths were 468 in North Dakota and 832 in South Dakota.
A natural experiment of sorts that provides some evidence in favor of the efficacy of mask mandates. Of course, we need more than once such experiment before we can draw firm conclusions.
The cases data themselves (which you so kindly shared) are interesting, and also offer support for mask mandates. They show a much bigger drop in cases in North Dakota following the implementation of the governor's mandate. Thank you for providing such an informative graph (whether intentionally or not).