Percentage Change vs. Percentage Points in COVID Mask Data

David Moore
3 min readMar 7, 2021

Last night on Twitter, I noticed that people were sharing this March 5 CDC report, portraying it incorrectly as evidence that masks were not effective.

Many people sharing the link said that it showed that masks “lowered cases by 1.5%,” suggesting that is not very much. That is not a correct representation.

(To be clear, yes, we should absolutely use masks and mask mandates toward public health goals and mitigate this terrible pandemic. See below.)

They were mistaking an overall percentage change (which is not the statistic in question) with a daily percentage point decrease (which was being reported).

Here’s Wikipedia on the difference, and here again on a student help guide. Or here:

The report states, “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…”.

The AP story about the CDC report last week writes, “‘Each day that growth rate is going down, the cumulative effect — in terms of cases and deaths — adds up to be quite substantial,’ said Gery Guy Jr., a CDC scientist who was the study’s lead author.”

Here are more online resources on the scientific consensus around the positive effects of masking. Nina Bai, science writer for UC San Francisco, summarized the CDC’s changes in guidance. A June 2020 study in Health Affairs found that “mask mandates led to a slowdown in daily COVID-19 growth rate.”

In January, writer Zeynep Tufecki and Jeremy Howard, researcher at the University of San Francisco a co-founder of the Masks4All campaign, wrote, “Taiwan massively scaled up its manufacturing of masks at the start of 2020, such that by April every citizen received a fresh supply of high-quality masks each week, and the distribution system was regulated by the government. Taiwan’s COVID-19 death rate per capita is more than 1,000 times lower than that in the U.S.”

Last month, Tufecki wrote, “Wearing masks, for example, posed few downsides, and held the prospect of mitigating the exponential threat we faced.”

Tufecki wrote, “After I wrote an article explaining how overdispersion and super-spreading were driving the pandemic, I discovered that this mechanism had also been poorly explained. I was inundated by messages from people, including elected officials around the world, saying they had no idea that this was the case. None of it was secret — numerous academic papers and articles had been written about it — but it had not been integrated into our messaging or our guidelines despite its great importance.

“Crucially, super-spreading isn’t equally distributed; poorly ventilated indoor spaces can facilitate the spread of the virus over longer distances, and in shorter periods of time, than the guidelines suggested, and help fuel the pandemic.”

With nearly 29 million known Covid cases in the U.S. as of this writing, a 2 percentage point drop daily throughout the past year could have reduced virus transmission, as is being observed with flu rates this past year.

Contact: on Twttr.



David Moore

Co-founder, investigative journalism on money in politics. Previously: OpenCongress, AskThem, Councilmatic.