Control limits

Control limits are horizontal lines drawn on an SPC control chart, usually at a distance of ±3 standard deviations from the mean of the plotted statistic.

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.

Control limits should not be confused with tolerance limits, which are completely independent of the distribution of the plotted sample statistic.