Silly Math, Silly Results, Bad Policy – The Most Recent CDC Review on Masks Mandates

The purpose of this post is simply to inform and point out how framing certain mathematical results can make them appear better than what they actually seem. The Morbidity and Mortality Weekly Report (MMWR) published on March 5, 2021 stated categorically that based on their analysis “[m]andating masks was associated with a decrease in daily COVID-19 case and death growth rates within 20 days of implementation.” One would think that making such categorical statement would have behind some mighty metric as support – one which the conclusion would be undeniable. But looking at the actual numbers reported in the MMWR, the matter is not as categorical as the previous statement led us to believe. 

Here is what the actual results reported in page 2:

Mask mandates were associated with a 0.5 percentage point decrease (p = 0.02) in daily COVID-19 case growth rates 1–20 days after implementation and decreases of 1.1, 1.5, 1.7, and 1.8 percentage points 21–40, 41–60, 61–80, and 81–100 days, respectively, after implementation (p<0.01 for all) (Table 1) (Figure).

In other words, mandating masks was associated with a 0.5% decrease in the growth of daily COVID-19 cases. That is not a typo, a 0.5%, less than a one percent difference. Granted, the reported metrics were deemed to be statistically significant with 95% confidence, which is another way to say that there is a 5% chance that the 0.5% value is wrong.  And when you take into account the error rate of that metric, which was reported to be about 1%, you effectively end up with inconclusive results. The same observation applies for the other time periods (i.e. 21-40, 41-60, etc.).

Lastly, the idea of studying the relationship between mask mandates and deaths is quite frankly bizarre. The association is not obvious nor a proven fact. COVID deaths are caused by COVID virus. Once you have the virus, mask or no mask, nature will run its course.