Improvement Insights Blog
p/u Charts or XmR Charts?
People ask: Should I use a p or u chart, or XmR chart? Which one is right? It depends…
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“Hi, I’m Jay Arthur, author of “Lean Six Sigma For Hospitals” and QI Macros [software].
“One of the things I’ve noticed is I get calls from people and they ask me, “Well, Jay, should we be using a p [chart] and u charts or an XmR chart of ratios?” And I can tell you: that depends on who your statistician leader is.
“If you’re looking at the Healthcare Data Guide, Lloyd Provost and his partner [Sandra Murray] love p and u charts… love them! They don’t like XmR charts. However, Dr. Donald Wheeler loves the XmR chart, which he calls ‘the Swiss Army knife of charts.’
“He says the reason is with p and u charts, you’re getting theoretical limits calculated based on theory of poisson and binomial and whatever. It tends to create more out-of-control points, so it gives you false alarms. He says [using] an XmR chart, we’ve just taken the numerator and the denominator and divided them and come up with a ratio. Guess what he calls that? Experiential data; that data is actually what’s really going on, and he thinks it charts better and doesn’t give you as many alerts.
“Now, this is one of the things that always concerns me: there’s no one right answer, right? You know, ‘Statistics means never having to say you’re certain’ is one of the quotes I always liked, right? So we have this challenge. [John] Tukey said that the plural noun of statistician is ‘a quarrel of statisticians’… a quarrel, because there’s all these debates back and forth.
“So what’s the right chart: a p and u chart or an XmR chart? Well, if you’re using either one, at least you’re doing that – a lot of people are just using line and bar charts. So, pick one and start using it, and I think you’ll find out a lot more about your process by using control charts than by not using control charts.
“That’s my Improvement Insight. Let’s go out and improve something this week.”
