Time-weighted charts
Control charts that take into account historical data, allowing detection
of relatively small shifts from the target value. For
example, a milled part has to fit precisely with another part. The milling
process is already in control. You can use a time-weighted chart to look
for small shifts in shape over time.
Minitab offers three time-weighted control charts:
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EWMA chart |
Moving average chart |
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A chart of exponentially weighted moving averages. EWMA charts can be
custom tailored to detect any size shift in the process and are not greatly
influenced when a small or large value enters the calculation. |
A chart of unweighted moving averages. EWMA charts are generally preferred
over MA charts because they weight the observations.
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CUSUM chart |
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A chart of the cumulative sum of the deviations from a nominal specification.
Good for detecting shifts in the mean that are 2 sigma or less. |
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EWMA, moving average, and CUSUM charts can be used with individual observations
or data in subgroups. Typically, you use these charts to evaluate the
process level. However, you can also use EWMA and CUSUM charts to plot
control charts for subgroup ranges or standard deviations to evaluate
the process variation.