Instead, a Moving Average, EWMA or Cu Sum chart for individuals can be used to verify statistical control. The Individual-X Chart, however, requires that we know the distribution of the process, since this will define the statistical control limits. When process distributions are bounded or skewed, or when Rational Subgroups cannot be formed, then it is better to use a chart designed for individuals (n=1) data. If rational subgroups can be formed, the X-bar Charts are generally preferred, since the control limits are easily calculated using the Normal distribution. If this is not the case, then trends or shifts in the process may not be detected, but instead attributed to random (common cause) variation. For this reason, you must have data that is time-ordered that is, entered in the sequence from which it was generated. ![]() The x-axes are time based, so that the charts show a history of the process. Each subgroup, consisting of a single observation, represents a "snapshot" of the process at a given point in time. ![]() Individual-X / Moving Range charts are generally used when you cannot group measurements into rational subgroups, when it is more convenient to monitor actual observations rather than subgroup averages, or when the process distribution is very skewed or bounded. An Individual-X Moving Range Chart with process capability estimates from SPC IV Excel software.
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