FIGURE 20-24. Control chart interpretation.
corrections, rather than letting the natural range of variability stabilize. The following paragraphs describe some of the ways the underlying distribution patterns can behave or misbehave.
Runs. When several successive points line up on one side of the central line, this pattern is called a run. The number of points in that run is called the length of the run. As a rule of thumb, if the run has a length of seven points, there is an abnormality in the process. Figure 20-25 demonstrates a run.
Trends. If there is a continued rise of all in a series of points, this pattern is called a trend. In general, if seven consecutive points continue to rise or fall, there is an abnormality. Often, the points go beyond one of the control limits before reaching seven. Figure 20-26 demonstrates a trend.
Periodicity. Points that show the same pattern of change (rise or fall) over equal intervals denote periodicity. Figure 20-27 demonstrates periodicity.
Hugging the centerline or control limit. Points on the control chart that are close to the central line, or to the control limit, are said to hug the line. Often, in this situation, a different type of data or data from different factors have been mixed into the subgroup. In such cases it is necessary to change the subgrouping, reassemble the data, and redraw the control chart. To decide whether there is hugging of the centerline, draw two lines on the control chart, one between the centerline and the UCL and the other between the centerline and the LCL. If most of the points are between these two lines, there is an abnormal-
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