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Give recent readings more weight
A basic Shewhart chart with a single-point rule compares each current statistic with its control limits. An exponentially weighted moving average (EWMA) updates a running value using a weighted combination of the new reading and the previous EWMA. Recent observations carry more weight, while older readings gradually have less influence.
Packaging sample showroom of SamBound New Material Technology Company Ltd. (sambound.com).
A small change can build across readings
Suppose a hypothetical packaging dimension has a baseline mean of 0.300 mm. Several successive readings are near 0.305 mm and each is within the individual Shewhart limits. The EWMA can move toward 0.305 mm as those readings accumulate. Whether it signals depends on the data, the weighting factor and the control limits chosen for the baseline.
An EWMA chart can improve sensitivity to small sustained changes under an appropriate design. A basic Shewhart chart remains useful for large, abrupt shifts. Establish the baseline and check for serial dependence before adopting either chart’s usual limits.
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