CNP

CNP (LET)

Let Subcommand

Compute the C_np process capability index for a variable.

This statistic computes the sample C_np process capability index of an industrial process and is defined as follows:
\( C_{np}= \frac{ENGUSL - ENGLSL} {P(0.99865) - P(0.00135)} \)

with \( P(x) \), ENGLSL and ENGUSL denoting the percentile function, lower engineering specification limit and the upper engineering specification limit, respectively. This statistic has coverage comparable to the normal-based C_p statistic for \( \pm 3 \sigma \). An alternative definition has 99% coverage and is defined as

\( C_{np}= \frac{ENGUSL - ENGLSL} {P(0.995) - P(0.005)} \)

This is a non-parametric alternative to the normal-based C_p process capability index.

The specification limits define the range within which a product is considered acceptable (values outside this range indicate that a product is defective).

LET <par> = CNP <y>             <SUBSET/EXCEPT/FOR qualification>
where <y> is the response variable;
            <par> is a parameter where the computed C_np is stored;
and where the <SUBSET/EXCEPT/FOR qualification> is optional.

LET A = CNP Y1
LET A = CNP Y1 SUBSET TAG > 2

The upper and lower specification limits must be specified by the user as follows:
LET LSL = <value>
LET USL = <value>

To use the alternative definition, enter the command
SET CNPK DEFINITION AIRFORCE

To reset the default, enter

SET CNPK DEFINITION PEARN

Dataplot statistics can be used in a number of commands. For details, enter
HELP STATISTICS

None

None

CAPABILITY ANALYSIS	=	Perform a capability analysis.
CP	=	Compute a Cp process capability index.
CPK	=	Compute a Cpk process capability index.
CNPK	=	Compute a Cnpk process capability index.
CPM	=	Compute a Cpm process capability index.
CPMK	=	Compute a Cpmk process capability index.
CC	=	Compute a Cc process capability index.
CPL	=	Compute a Cpl process capability index.
CPU	=	Compute a Cpu process capability index.
CNPM	=	Compute a Cnpm process capability index.
CNPKM	=	Compute a Cnpkm process capability index.
PERCENT DEFECTIVE	=	Compute the percentage of defectives in a sample.
EXPECTED LOSS	=	Compute the expected loss of a sample.

Kotz and Johnson (1993), "Process Capability Indices," Chapman & Hall (chapter 2).

Chen and Ding (2001), "A New Process Capability Index for Non-Normal Distributions," International Journal of Quality & Reliability Management, Vol. 18, No. 7, pp. 762-770.

Quality Control

2015/04

 
SKIP 25
READ FURNACE.DAT X1 X2 X3 Y
LET LSL = 460
LET USL = 660
LET A = CNP Y 
MULTIPLOT CORNER COORDINATES 5 5 95 95
MULTIPLOT 2 2
MULTIPLOT SCALE FACTOR 2
TITLE AUTOMATIC
CNP PLOT Y X1
CNP PLOT Y X2
CNP PLOT Y X3
END OF MULTIPLOT