What the Heck is a Shewhart Chart?

This is just a post from a retired psychiatrist who barely passed the statistics course in medical school, so take it with a big grain of salt. I found an article written by Rocco J. Perla about something called Shewhart control charts to monitor the course of pandemic fluctuations. I noticed it because of what Perla commented on, which is the tendency of the press to make sensational headlines about every squeak in the turning wheel of COVID-19, including CDC making changes in masking guidelines leading to congressmen calling for investigations of the CDC for making the changes. It makes it look as though we don’t know what we’re doing.

I don’t know if Shewhart charts can help us make better decisions about what to advise us to do at the community level to help monitor and predict outbreaks. But it looks like we need to try something better soon. Because I can’t stand the pop-up ads at the U.S. News & World Report web site and limitations on how many articles I can view for free where I first saw Perla’s story, I looked up his original article published in the Int J Qual Health Care.

This led to my discovering the web site ISQua (International Society for Quality in Health Care). There I found a Shewchart for my state of Iowa, which shows what happened here this month. I don’t know if the Iowa Department of Public Health (IDPH) is already using it and I’m not qualified to tell them what to do. By the way, I think that mindset of “Don’t tell me what to do” crankiness might be an epiphenomenon of the COVID-19 pandemic. Nobody wants to be told to wear masks or to get a vaccine. It just leads to a pandemic of backlashes. I don’t know if Rocco is right about the Shewhart chart method not being in use by public health officials. But I want to go on record as not telling anybody what to do.

The Shewhart charts look labor intensive and maybe that’s why some public health departments don’t use them. They’re understaffed and overworked. Iowa has been scaling back the collection and reporting of COVID-19 data, partly because things appeared to be so rosy early in July. I’m not so sure how rosy they’ll be after the Iowa State Fair in August, which is expected to draw about a million visitors.

But I’m not telling anyone not to go to the state fair and I’m not telling anyone to wear a mask and I’m not telling anyone to get vaccinated. I’m not even telling anyone to pay any attention to Perla’s article on the Shewhart chart. I am telling you that I’m too old and cranky to be getting backlashes about any of this business.

Have fun at the fair.

Perla RJ, Provost SM, Parry GJ, Little K, Provost LP. Understanding variation in reported covid-19 deaths with a novel Shewhart chart application. Int J Qual Health Care. 2021;33(1): mzaa069. doi:10.1093/intqhc/mzaa069

Inkelas M, Blair C, Furukawa D, Manuel VG, Malenfant JH, Martin E, et al. (2021) Using control charts to understand community variation in COVID-19. PLoS ONE 16(4): e0248500. https://doi.org/10.1371/journal.pone.0248500

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