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In order to judge if a process is in control, X̄ & R charts should be examined together and the process should be deemed in statistical control if both the charts show a state of control. Situations exists where R-chart is in a state of control but X̄ is not.
Given below are such situations:
In order to judge if a process is in control, X̄ & R charts should be examined together and the process should be deemed in statistical control if both the charts show a state of control. Situations exists where R-chart is in a state of control but X̄ is not.
Given below are such situations:
Serial #
|
R-chart
|
X̄
|
Interpretation
|
| 1 | In control | Points beyond limits only on one side | Level of process has shifted |
| 2 | In control | Points beyond limits on both the side | Level of process is changing in erratic manner –frequent adjustments. |
| 3 | Out of control | Points beyond limits on both the side | Variability has increased |
| 4 | Out of control | Out of control on one side | Both level and variability has changed |
| 5 | In control | Run of 7 or more points on one side of central line | Shift in process level |
| 6 | In control | Trend of 7 or more points. No point outside control limits | Process level is gradually changing |
| 7 | Run of 7 or more points above central line. | --- | Variability has increased |
| 8 | Points too close to the central line | -- | Systematic difference within sub-groups |
| 9 | ---- | Points too close to the central line | Systematic differences within sub-groups. |
Source: Fundamentals of Applied Statistics, Gupta & Kapoor.
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