<P> The most important principle for choosing a set of rules is that the choice be made before the data is inspected . Choosing rules once the data have been seen tends to increase the Type I error rate owing to testing effects suggested by the data . </P> <P> In 1935, the British Standards Institution, under the influence of Egon Pearson and against Shewhart's spirit, adopted control charts, replacing 3 - sigma limits with limits based on percentiles of the normal distribution . This move continues to be represented by John Oakland and others but has been widely deprecated by writers in the Shewhart--Deming tradition . </P> <P> When a point falls outside the limits established for a given control chart, those responsible for the underlying process are expected to determine whether a special cause has occurred . If one has, it is appropriate to determine if the results with the special cause are better than or worse than results from common causes alone . If worse, then that cause should be eliminated if possible . If better, it may be appropriate to intentionally retain the special cause within the system producing the results . </P> <P> Even when a process is in control (that is, no special causes are present in the system), there is approximately a 0.27% probability of a point exceeding 3 - sigma control limits . So, even an in control process plotted on a properly constructed control chart will eventually signal the possible presence of a special cause, even though one may not have actually occurred . For a Shewhart control chart using 3 - sigma limits, this false alarm occurs on average once every 1 / 0.0027 or 370.4 observations . Therefore, the in - control average run length (or in - control ARL) of a Shewhart chart is 370.4 . </P>

How many types of variation are there in a control chart