Once the limits are drawn, reading the chart means looking for patterns that would be unlikely if the process were running steadily. The clearest signal is a point beyond a control limit. When the process is stable, a subgroup average falling outside the limits is rare enough that it is worth stopping to ask why.
Points can stay inside the limits and still tell you something has shifted. A run of seven consecutive points on the same side of the centreline means the process centre has probably moved. A trend of seven points steadily rising or steadily falling means something is drifting, such as a glue nozzle wearing or a web tension creeping. Both patterns are signals even though no single point is outside.
Read the R chart first. If the range chart shows a signal, the spread within subgroups has changed, and the X̄ limits, which were built from R̄, are no longer trustworthy. If the R chart is quiet and the X̄ chart signals, the spread is steady but the centre has moved. Knowing which chart spoke points you toward the right kind of cause.
Standard Procedure
- 1Check the R chart for any point above UCL for R.
- 2Check the X̄ chart for any point beyond either limit.
- 3Count consecutive points on one side of the centreline; seven or more is a signal.
- 4Count consecutive points moving in one direction; seven or more is a signal.
- 5Mark any signal on the chart and log the time so the cause can be traced.
Lesson Summary
- ✓Points beyond limits, runs of seven on one side, and trends of seven are the basic signals.
- ✓Read the R chart first; an unstable range undermines the X̄ limits.