BS ISO 7870-6:2016 pdf free
BS ISO 7870-6:2016 pdf free.Control charts – EWMA control charts
BS ISO 7870-6 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 lttle depending upon the smoothing parameter (2).
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
一production rate is slow,
一sampling and inspection procedure is complex and time consuming,
一testing is expensive, and
一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).
a) The EWMA control chart is sensitive to small shifts in the process mean, but does not reflect the out of control situations for larger shifts as quickly as the Shewhart control chart does. It is also recommended to superimpose the EWMA control chart on top of a suitable Shewhart control chart with widened control limits in order to detect both small and large shifts in the process mean.
b) When EWMA control chart is used with a small value of the weight λ then at the beginning of the production, the EWMA control chart is more efficient in detecting the shift. As production proceeds,if a trend develops, the trend is unfortunately shown to be well within control of the EWMA chart’s control limits.BS ISO 7870-6 pdf download.
