Note: The control limits are typically referred to as 3 sigma limits, using a Normal distribution approximation. Indicator lines may be drawn at ± 1 sigma and ± 2 sigma, which equal 1/3 and 2/3, respectively, of the difference, between the centerline and the control limit values. These 1, 2, and 3 sigma values are used when applying the Tests for Special Cause variation described in Appendix E.