Question

When using a control chart, what are some patterns that would indicate that the process is out of control? Additionally explain what might cause a process to be out of control. In your responses to other students, relate and identify to an example of process that you have seen out of control.

Answer 1

In statistics, Control charts are the tools in control processes to determine whether a manufacturing process or a business process is in a controlled statistical state. This chart is a graph which is used to study process changes over time. The data is plotted in a timely order. It is bound to have a central line of average, an upper line of upper control limit and a lower line of lower control limit. In addition, the data obtained from the process can also be applied in making the prediction of the future performances of the process.

When the analysis made by the control chart indicates that the process is currently under control, it reveals that the process is stable with the variations that come from sources familiar with the process. No changes or corrections are required to be made to the parameters of process control.

If analysis of the control chart indicates that the process is currently under control (i.e., is stable, with variation only coming from sources common to the process), then no corrections or changes to process control parameters are needed or desired. In addition, data from the process can be used to predict the future performance of the process. If the chart indicates that the monitored process is not in control, analysis of the chart can help determine the sources of variation, as this will result in degraded process performance.[1] A process that is stable but operating outside desired (specification) limits (e.g., scrap rates may be in statistical control but above desired limits) needs to be improved through a deliberate effort to understand the causes of current performance and fundamentally improve the process.

Rule	Rule Name	Pattern
1	Beyond Limits	One or more points beyond the control limits
2	Zone A	2 out of 3 consecutive points in Zone A or beyond
3	Zone B	4 out of 5 consecutive points in Zone B or beyond
4	Zone C	7 or more consecutive points on one side of the average (in Zone C or beyond)
5	Trend	7 consecutive points trending up or trending down
6	Mixture	8 consecutive points with no points in Zone C
7	Stratification	15 consecutive points in Zone C
8	Over-control	14 consecutive points alternating up and down

Pattern Description	Rules	Possible Causes
Large shifts from the average	1, 2	New person doing the job Wrong setup Measurement error Process step skipped Process step not completed Power failure Equipment breakdown
Small shifts from the average	3, 4	Raw material change Change in work instruction Different measurement device/calibration Different shift Person gains greater skills in doing the job Change in maintenance program Change in setup procedure
Trends	5	Tooling wear Temperature effects (cooling, heating)
Mixtures	6	More than one process present (e.g. shifts, machines, raw material.)
Stratifications	7	More than one process present (e.g. shifts, machines, raw materials)
Over-control	8	Tampering by operator Alternating raw materials

Figure 1 Control Chart: Out-of-Control Signals

• A single point outside the control limits. In Figure 1, point sixteen is above the UCL (upper control limit).
• Two out of three successive points are on the same side of the centerline and farther than 2 σ from it. In Figure 1, point 4 sends that signal.
• Four out of five successive points are on the same side of the centerline and farther than 1 σ from it. In Figure 1, point 11 sends that signal.
• A run of eight in a row are on the same side of the centerline. Or 10 out of 11, 12 out of 14, or 16 out of 20. In Figure 1, point 21 is eighth in a row above the centerline.
• Obvious consistent or persistent patterns that suggest something unusual about your data and your process.