# Middle Third Rule

The Middle Third control check is violated for the mean chart whenever more than two thirds of the mean of the sample just collected and the means of all previous samples collected are within a one sigma spread about the center line of the mean chart. The Middle Third control check is violated for the range chart whenever more than two thirds of the mean of the sample just collected and the ranges of all previous samples collected are within a one sigma spread about the center line of the range chart. The spread between the current upper and lower control limits of the mean and range chart is 6 sigma wide; therefore the plus and minus one sigma spread about the centerline designates the middle third region of a control chart.