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2.
Measurement Process Characterization
2.2. Statistical control of a measurement process 2.2.3. How is short-term variability controlled?
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| Monitoring future precision | Once the base line and control limit for the control chart have been determined from historical data, the measurement process enters the monitoring stage. In the control chart shown below, the control limit is based on the data taken prior to 1985. | ||
| Each new standard deviation is monitored on the control chart | Each new short-term standard deviation based on J measurements is plotted on the control chart; points that exceed the control limits probably indicate lack of statistical control. Drift over time indicates degradation of the instrument. Points out of control require remedial action, and possible causes of out of control signals need to be understood when developing strategies for dealing with outliers. | ||
| Control chart for precision for a mass balance from historical standard deviations for the balance with 3 degrees of freedom each. The control chart identifies two outliers and slight degradation over time in the precision of the balance |
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| Monitoring where the number of measurements are different from J |
There is no requirement that future standard deviations be based on
J, the number of measurements in the historical database.
However, a change in the number of measurements leads to a change in the
test for control, and it may not be convenient to draw a control chart
where the control limits are changing with each new measurement sequence.
For a new standard deviation based on J' measurements, the precision of the instrument is in control if $${\large s}_{new} < {\large s}_1 \sqrt{F_{\alpha, \, J'-1, \, K(J-1)}}$$ Notice that the numerator degrees of freedom, v1 = J' - 1, changes but the denominator degrees of freedom, v2 = K(J - 1), remains the same. |
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