Control limits are horizontal lines drawn on an statistical process control chart, usually at a distance of ±3 standard deviations of the plotted statistic from the statistic's mean.
For normally distributed statistics, the area bracketed by the control limits will on average contain 99.73% of all the plot points on the chart, as long as the process is and remains in statistical control.
Reference
http://en.wikipedia.org/wiki/Control_limits
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