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Step 2: Evaluate Stability
Objective:
Determine if the measurement system is stable (in statistical control)
over time.
Process:
Either choose:
a. one sample standard. If one is not available, use a production unit
whose measurement falls into the middle of the expected
measurement range.
or
b. several sample standards from the low end, middle, and high end
of the expected measurement range. If they are not available,
then use production units. If several samples are chosen,
separate control charts and data analyses will need to be
performed.
NOTE: The exact value of these samples is not critical for this
evaluation, but would help if these samples are to be used for other
studies.
Measure the sample standard three to five times in as short a period
of time as possible to obtain one group of data. This should be
performed on a regular basis, such as hourly, daily, or weekly.
Initially, 25 groups of data should be gathered.
Plot the data on an Average (
X
) control chart, pronounced “x bar”,
and Range (R) control chart with their appropriate control limits.
