Understanding ASQ-CQA -Knowing The Ways

If ũ and σ are estimated to be 98.94 μm and 1.03 μm, respectively, then
C p = USL−LSL
6σ
= 106.00−94.00
6 x 1.03
= 1.94
Index
C pk = min[ USL−ũ , ũ−LSL ] = min[ 106.00−98.94 , 98.94−94.00
3σ 3σ
] = 1.6
3 x 1.03 3 x 1.03
C
C pm =
p
√[1+( ũ−T
σ )2 ] = 1.94
√[1+( 98.94−100.00
1.03 )2 ] = 1.35
C pkm =
C pk
√[1+( ũ−T
σ )2 ] = 1.6
√[1+( 98.94−100.00
1.03 )2 ] = 1.11
The fact that the process is running off-center (about 1σ below its target) is reflected in
the markedly different values for C p , C pk , C pm , and C pkm .
C pk = min[ USL−ũ
3σ , ũ−LSL
3σ
C pkm =
] = min[ 106.00−98.94
, 98.94−94.00
3 x 1.03 3 x 1.03
C pk
√[1+( ũ−T
σ )2 ] = 1.6
√[1+( 98.94−100.00
1.03 )2 ] = 1.11
] = 1.6
Charlie Chong/ Fion Zhang