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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Case study: Resistivity
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A schedule should be set up for making measurements with a
single instrument (once a day, twice a week, or whatever is
appropriate for sampling all conditions of measurement).
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Short-term standard deviations
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The measurements are denoted
where there are J measurements on each of K occasions.
The average for the kth occasion is:
The short-term (repeatability) standard deviation for the
kth occasion is:
with (J-1) degrees of freedom.
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Pooled standard deviation
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The repeatability standard deviations are pooled over the K
occasions to obtain an estimate with K(J - 1) degrees of
freedom of the level-1 standard deviation
Note: The same notation is used for the repeatability standard deviation
whether it is based on one set of measurements or pooled over several
sets.
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Database
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The individual short-term standard deviations along with
identifications for all significant factors are recorded
in a file. The best way to record this information is by using
one file with one line (row in a spreadsheet) of information
in fixed fields for each group. A list of typical entries follows.
- Identification of test item or check standard
- Date
- Short-term standard deviation
- Degrees of freedom
- Instrument
- Operator
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