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Glossary of Terms - Definitions
The following process indices, Cp, Cpk, Pp & Ppk are defined by the Statistical Process Control Reference Manual, 2nd edition, 2005, put out by
AIAG. These indices are only valid when the process is stable or under statistical control.
Cp, Cpk, Pp & Ppk should always be evaluated together. A Cp value significantly greater than its corresponding Cpk value, or a Pp value signifi-
cantly greater than its corresponding Ppk value indicates possible improvement by centering the process.
If the process is in statistical control, the C and P indices will be very close to each other. A large difference between the C and P indices indicate
the presence of variation caused by special causes which affects only some of the process output.
Cp
This is a process capability index which provides a measure of how well the process will satisfy the variability requirements but does not take
into account how well centered the process is. It compares the Within-Subgroup Variation, σ
by the Upper & Lower Specification Limits (USL & LSL). Cp can be calculated for two-sided (bilateral) tolerances only and since the within-sub-
group variation is based on a subgroup size of 5, the total number of readings must be a multiple of 5, and must be under 200 total readings.
Cp may range in value from 0 to infinity. A large value indicates greater potential capability and a value of 1.33 or greater is desirable.
(USL-LSL)
Cp =
6*(σ
)
C
Cpk
This is a process capability index which provides a measure of how well the process will satisfy the variability requirements and takes into
account how well centered the process is. For bilateral tolerances Cpk is always less than or equal to Cp, and if the process is perfectly centered,
then Cpk will have a value equal to Cp. Due to the Within-Subgroup Variation σ
multiple of 5, and must be under 200 total readings. Cpk may be negative, indicating the process mean is outside the tolerance limits, or may
range in value from 0 to infinity. A large value indicates greater potential capability and a value of 1.33 or greater is desirable.
(USL-X) or (X-LSL)
Cpk = lesser of
3*(σ
)
C
Pp
This is a process performance index which provides a measure of how well the process satisfied the variability requirements but does not take
into account how well centered the process is. It compares the Total Process Variation, σP, to the maximum allowable variation as indicated by
the Upper & Lower Specification Limits (USL & LSL). Pp can be calculated for two-sided (bilateral) tolerances only. Pp may range in value from 0
to infinity. A large value indicates greater process performance and a value of 1.33 or greater is desirable.
Pp = (USL-LSL)
6*(σ
)
P
Ppk
This is a process performance index which provides a measure of how well the process satisfied the variability requirements and takes into
account how well centered the process is. For bilateral tolerances Ppk is always less than or equal to Pp, and if the process is perfectly centered,
then Ppk will have a value equal to Pp. Ppk may be negative, indicating the process mean is outside the tolerance limits, or may range in value
from 0 to infinity. A large value indicates greater potential capability and a value of 1.33 or greater is desirable.
(USL-
)
X
Ppk = lesser of
or
3*(σ
)
P
CAM
This is an alternative process capability index, similar to Cp, but takes into account both subgroup size and total number of subgroups. CAM
provides a measure of how well the process will satisfy the variability requirements but does not take into account how well centered the
process is. It compares the Within-Subgroup Variation, σ
tion Limits (USL & LSL). CAM can be calculated for two-sided (bilateral) tolerances only and since the within-subgroup variation is based on a
subgroup size of 5, the total number of readings must be a multiple of 5, and must be under 200 total readings. CAM may range in value from 0
to infinity. A large value indicates greater potential capability and a value of 1.5 or greater is desirable.
(USL-LSL)
CAM =
6*(σ
)
CAM
CMK
This is an alternative process capability index, similar to Cpk, but takes into account both subgroup size and total number of subgroups. CMK
provides a measure of how well the process will satisfy the variability requirements and takes into account how well centered the process is. For
bilateral tolerances CMK is always less than or equal to CAM, and if the process is perfectly centered, then CMK will have a value equal to CAM.
Due to the Within-Subgroup Variation σ
total readings. CMK may be negative, indicating the process mean is outside the tolerance limits, or may range in value from 0 to infinity. A
large value indicates greater potential capability and a value of 1.3 or greater is desirable.
(USL-
) or (
X
CMK = lesser of
3*(σ
)
CAM
d2
A divisor of R used to estimate the process standard deviation or Within-Subgroup Variation, σ
size of 5 is 2.326.
45652906_ed1
3*(σ
)
C
(
- LSL)
X
3*(σ
)
P
CAM
, CMK is also only valid where the total number of readings is a multiple of 5, and must be under 200
CAM
- LSL)
X
3*(σ
)
CAM
, Cpk is also only valid where the total number of readings is a
C
, to the maximum allowable variation as indicated by the Upper & Lower Specifica-
, to the maximum allowable variation as indicated
C
, for Cp & Cpk. The value of d2 for a subgroup
C
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