Joan asked me post this on her behalf because she is traveling this week.
Once upon a time histograms, bar charts, and line graphs were the most important tools for displaying infection control data. We used them to show differences between events and to track changes over time.
Now someone has decided that control charts are essential tools for measuring success in healthcare. We spend a lot of time constructing charts that show upper and lower confidence limits. We faithfully plot our own results on them and hope that we will not meet the criteria for being “out of control”.
We have rules for what it means to be out of control: one spike above two standard deviations, a series of X in a row above or below the central line, or a series of ups and downs. How many of us have been called to task because a dreaded spike has appeared in our data.
Would it surprise you to know that control charts are business tools? They were not introduced into health care by epidemiologists. They were adopted by people in healthcare who had business backgrounds. We have adopted them because they seem like better tools than our old stand-bys.
What are we really comparing ourselves to when we use control charts? The goal of control charts is to keep a process average. The center line represents average performance and, in business, the goal is to keep it there.
In healthcare we are always striving to be better. In most cases this means that we want the lines to move steadily up or down. We want to have more of what is good and less of what is bad. So why are we caught up in control charts?
This month I am linking you to a business consulting website that explains control charts. Being a business site, the explanations are about applying control charts to the production of widgets. Notably absent are recommendations to use control charts to measure complex healthcare processes that involve not just machinery, but the unpredictability of people (both patients and workers).
I think it is time that we move away from the model of infection prevention as a business. A big step in doing so is to use statistics that correctly show what we really are all about.
Please let me know if there are any questions I can answer about these articles.
Individual control charts: different types of control charts, what they tell us and when to use them. Available here.
An example of misuse of statistical process control in healthcare.
An example of overcontrolling a process: Over-reacting to points on a control chart that are above or below a confidence level.