u-Charts

Hints on How to Read the Charts

The most obvious interpretation is when one or more data points fall outside your control limits (either upper or lower). Those values should be investigated for assignable causes, and the assignable causes should be fixed. If your control chart shows three out of four consecutive points hovering closer to the limits than to the centerline, this pattern may signal a shift or trend, and should be investigated (because predictable processes generally show 85 to 90 percent of the data closer to the centerline than to the limits). Remember: useful limits can be constructed with as few as five or six consecutive values. However, the more data used to compute the limits, the greater the certainty of the results.

Another way to spot trends is to look at the data points along the centerline. If eight or more consecutive data points are clustered on the same side of the centerline, a shift in the original baseline or performance of the process has probably occurred, even without a data point falling outside the limits. This is a signal to be investigated.

Also look for six consecutive points, either increasing or decreasing in value. These points could signal a trend. Other points to notice are 14 points in a row alternating up and down, up and down; or 15 points clustered around the center line, with some of the points immediately above the line, and some immediately below the line.

Notice that we do not say that "this is a problem." We do not know yet. However, we do know that something has happened that needs more focus.