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ISO DIS 7870-6

2014 Edition, January 27, 2014

Complete Document

STATISTICAL METHODS IN PROCESS MANAGEMENT - CONTROL CHARTS - PART6: EWMA CONTROL CHARTS



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Superseded By: ISO 7870-6

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NOW A PUBLISHED STD * SEE ISO 7870-6
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Description / Abstract:

This International standard covers EWMA charts as a statistical process control technique to detect small shifts in the process mean. It makes possible faster detection of small to moderate shifts in the process average. In this chart, the process average is evaluated in terms of exponentially-weighted moving average of current and all prior sample means. EWMA weights samples in geometrically decreasing order so that the most recent samples are weighted most highly while the most distant samples contribute very little.

The basic objective is therefore the same as that of the Shewhart control chart described in ISO 7870-2.

Its application is worthwhile in the rare situations when:

Statistical methods in process management — Control charts — Part 6:

- production rate is slow

- sampling and inspection procedure is complex and time consuming

- testing is expensive

- it involves safety risks

The EWMA is used extensively in time series modeling and in forecasting.

Variable control charts can by constructed for individual observations taken from the production line, rather than samples of observations. This is sometimes necessary when testing samples of multiple observations would be too expensive, inconvenient, or impossible. For example, the number of customer complaints or product returns may only be available on a monthly basis; yet, one would like to chart those numbers to detect quality problems. Another common application of these charts occurs in cases when automated testing devices inspect every single unit that is produced. In that case, one is often primarily interested in detecting small shifts in the product quality (for example, gradual deterioration of quality due to machine wear).
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