Improvement Insights Blog
Use XmR Charts instead of c, np, p and u Charts
I have found that an XmR chart is the easiest way to display attribute data. Simply convert the numerator/denominator into a ratio and plot the ratio.
- defects per day could be a c chart, but an XmR chart works just as well
- defects/samplesize could be np, p or u chart, but XmR chart works just as well using the ratio
Almost two decades ago, Tom Pyzdek said: X chart provides an excellent approximation to the p chart.
More recently, Donald Wheeler noted that XmR chart limits will be very close to c, np, p or u chart limits if the underlying distribution is correct. Wheel close with: You can guarantee that you have the right limits for your count-based data by simply using the XmR chart to begin with.
If you have some c, np, p and u data, you could create different charts for each type of data or you could convert it into ratios and use the QI Macros XmR Dashboard to monitor all of the charts.
I have also found that varying limits in p and u charts cause confusion in the audience. Then I have to explain why they go up and down. They lose track of what the data is telling them and get bogged down in trying to understand how the limits are calculated. Not so with the XmR. I just have to explain that the R chart shows the difference between each two data points.
Consider using the XmR chart for attribute data. It will simplify your life.
