Control charts - EWMA Control Charts

Control charts - EWMA Control Charts

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1   Scope

This International Standard covers EWMA control charts as a statistical process control technique to detect small shifts in the process mean. It makes possible the 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 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 depending upon the smoothing parameter (λ).

NOTE 1 The basic objective is the same as that of the Shewhart control chart described in ISO 7870‑2.

The Shewhart control chart’s application is worthwhile in the rare situations when
  • — production rate is slow,
  • — sampling and inspection procedure is complex and time consuming,
  • — testing is expensive, and
  • — it involves safety risks.

NOTE 2 Variables control charts can be 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).