Applying Statistical Process Control charts to sports injury research

The first article published from my Ph.D., explored for the first time, injury incidence rates in the three main domestic cricket competition formats in England and Wales (First-Class, One-Day, and T20) and described the epidemiology of cricket injuries across nine seasons from 2010 to 2018.

It was a privilege to have such rich data to work with that had never been fully formally analysed to that point. Having data over such a time period enabled us to use Statistical Process Control (SPC) charts, which were invented by Walter Shewhart working for Bell Labs in the 1920s as a way to help engineers reduce variation in the manufacturing process. Since then, they have been applied in manufacturing, business, military, and health, as a quantitative monitoring tool to detect statistically significant changes over time.

The use of supplementary ‘signaling’ rules (the most common of which were originally proposed in the Western Electric Handbook in 1956) with the charts, can highlight the need for further investigation when a supplementary rule has been met. These rules are:

- One or more points outside of the calculated control limits

- Two out of three consecutive points beyond two SD from the baseline

- Four out of five consecutive points beyond one SD from the baseline

- Nine consecutive points on one side of the historical baseline

With enough data, it allows for the identification of special variations from a particular data point’s own historic baseline. So, with enough data at our disposal, we were excited to apply these techniques for the first time in cricket injury research, to detect trends in match injury incidence for each competition format over the nine seasons.

The output is shown below (A — One-Day; B — T20; C — First-Class cricket) and although none of the supplementary ‘signaling’ rules were fulfilled, the charts did illustrate the consistency in match injury incidence between 2010 and 2018, suggesting the relative risk for each competition format was stable during this period.

I remember as I was presenting this data at the International Cricket Council World Congress in 2019, it occurred to me that the results could be perceived as being pretty unremarkable. No statistical significance, no supplementary rules had been met, no major scientific breakthroughs, just stable and consistent data. Unremarkable. But, as I tried to tell those in attendance, I realised the output was that unremarkable, it actually made it quite remarkable (Stick with me on this)… Because like Walter Shewart nearly a century ago, we had now established a baseline from which all future injury prevention initiatives could be continually monitored and visualised against, thanks to SPC charts.