A free online reference for statistical process control, process capability analysis, measurement systems analysis, control chart interpretation, and other quality metrics.
Notice how most of the points in the chart shown below are close to one control limit or the other. This pattern may indicate that subgroups have been drawn from two sources and the data has been mixed—for example, from two machines, two processes, or from two shifts. If this is the case, stratify (separate) the data and re-plot on two charts, or resolve the differences. If the data is not from two sources, the chart may indicate that overcontrolling or tampering is occurring. That is, the process or system is being constantly changed, causing the process to have increased variation.
See also: >> Any nonrandom pattern >> Too close to the average >> Cycles >> Trends >> Clusters >> Sawtooth >> 2 of 3 points beyond 2 sigma >> 4 of 5 points beyond 1 sigma