CPM

CPM (LET)

Let Subcommand

Compute the process capability index (CPM) for a variable.

The process capability index measure the performance (i.e., the capability) of an industrial process. The CPM is a variant of the CP and CPK capability indices.
\( CPM = \frac{\mbox{USL} - \mbox{LSL}} {6 \sqrt{s^2 + (\bar{x} - \mbox{TARGET})^2}} \)

where USL and LSL are user specified upper and lower specification limits, TARGET is the target engineering limit, and s are the sample mean and standard deviation of the data. For this statistic, larger is better.

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

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

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

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

None

None

CONTROL CHART	= Generate a control chart.
STATISTIC PLOT	= Generate a statistic versus subset plot.
DEX ... PLOT	= Generate a dex <statistic> plot.
CP	= Compute the process capability index.
CPK	= Compute the process capability index.
CNPK	= Compute the process capability index.
CC	= Compute the process capability index.
PERCENT DEFECTIVE	= Compute the percentage of defectives in a sample.
EXPECTED LOSS	= Compute the expected loss of a sample.

Kaoru Ishikawa (1982), "Guide to Quality Control," Asian Productivity Organization, (chapter 13).

Quality Control

1998/12

SKIP 25
READ GEAR.DAT Y X
LET LSL = 0.995
LET USL = 1.005
LET TARGET = 1.0
CHARACTER X BLANK
LINE BLANK SOLID
TITLE AUTOMATIC
CPM PLOT Y X