p Chart and u Chart Formats

p and u chart Control Limit Options in QI Macros

p charts and u charts can have upper control limits (UCL) and lower control limits (LCL) that vary with the sample size. QI Macros provides users with options about how these control limits are calculated and displayed.

Flat or skyline limits without 1 and 2 sigma lines (default)

When you run a p chart or u chart in QI Macros, the default format is "flat or skyline limits" that display the UCL, CL, LCL and your data.

p chart with flat control limits

How to Choose Between Flat (Skyline) and Wavy Limits

To change the QI Macros default from flat/skyline limits to wavy limits, click on Control Charts, then Control Chart Rules, then p/u UCL_LCL format.

control chart format options in QI Macros software

A window will open showing the new format you are selecting. Click OK to change the format.

control limit line options in QI Macros software

Now run the p chart or u chart again and the control limits will be wavy vs flat/skyline.

Here is a p chart with wavy control limits:

p chart with wavy control limits


Flat Horizontal Lines When There is Little Variation in the Sample Size

If there is little variation in the sample size, the p chart and u charts will have horizontal control limits. Here is the formula:

p chart  formula

p chart with horizontal control limits

p Chart UCL Limited to 100% Since it is a Fraction Defective Chart

Since the p chart is a fraction defective chart, the UCL is limited to 100% and the LCL is limited to 0%

p chart with UCL ceiling of 100%

p Chart Calculations for Small Sample Sizes

In certain cases, the standard formulas for control limits on the p chart cannot be used. This condition exists when:

n × pbar < 5

QI Macros p chart template contains a tab for these instances: p for np 5



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