Six Sigma & Process Variation

For a normal distribution:
-Approx 68% of variation is contained within +/- 1sigma
-Approx 95% of variation is contained within +/- 2sigma
-Approx 99.7% of variation is contained within +/- 3sigma

Cp = 1 when +/- 3 sigma is contained within spec limits.
Cp = 1.33 when +/- 4 sigma is contained within spec limits.
Cp = 1.50 when +/- 4.5 sigma is contained within spec limits.
Cp = 1.67 when +/- 5 sigma is contained within spec limits.
Cp = 2.00 when +/- 6 sigma is contained within spec limits.