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the means, mean of the ranges, mean of the proportions) - or for a reference period against which change can be assessed. Similarly a median can be used instead.
* A centre line is drawn at the value of the mean or median of the statistic
* The standard deviation (e.g., sqrt(variance) of the mean) of the statistic is calculated using all the samples - or again for a reference period against which change can be assessed. in the case of XmR charts, strictly it is an approximation of standard

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