u Chart in Excel

Use a u chart when counting defects and when the sample size varies

The purpose of a u chart is to determine stability of "counted" data (e.g., errors per bill, dents in a car door, etc.) when there can be more than one defect per unit and the sample size varies.

Defects (u charts) vs Defective Items (p charts):

  • Defects - each item in a sample can have more than one defect (e.g. dents on a car door, errors on a bill, missing products in a shipment).
  • Defective items: Each item is only counted once (e.g. car doors that need to be repainted, bills that are wrong, incomplete shipments, C-sections in a delivery),

u chart data looks like this:

example of u chart data

To create a u chart in Excel using QI Macros:

  1. Just click and drag over the data to select it.
  2. Then click on the QI Macros menu, Control Charts, then u chart.
  3. QI Macros will do all the math and draw the u chart for you. QI Macros will also turn any unstable points or conditions on the u chart red. These should be investigated.

Example of a u chart created using QI Macros for Excel

The stair step or skyline control limits are the result of the varying sample sizes.

u chart in Excel created with QI Macros software

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To create an U Chart using QI Macros...
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