# Sample C Control Chart

Graphics Commands C CONTROL CHART

DATAPLOT Reference Manual March 3, 1997 2-43

C CONTROL CHART

PURPOSE

Generates a (Poisson) counts control chart.

DESCRIPTION

A C chart is a data analysis technique for determining if a measurement process has gone out of statistical control. The C chart is

sensitive to changes in the number of defective items in the measurement process. The “C” in C CONTROL CHART stands for

“counts” as in defectives per lot. The C control chart consists of:

Vertical axis = the number defective for each sub-group;

Horizontal axis = sub-group designation.

The C chart assumes that each sub-group has an equal sample size (this sample size does not need to be speciﬁed). A sub-group is

typically a time sequence (e.g., the number of defectives in a daily production run where each day is considered a sub-group). If the

times are equally spaced, the horizontal axis variable can be generated as a sequence (e.g., LET X = SEQUENCE 1 1 N where N is the

number of sub-groups).

In addition, horizontal lines are drawn at the mean number of defecti ves and at the upper and lo wer control limits. The control limits are

calculated as:

(EQ 2-4)

(EQ 2-5)

where

c is the mean number of defectives. Also, zero serves as a lower bound on the LCL.

SYNTAX

C CONTROL CHART <y1> <x> <SUBSET/EXCEPT/FOR qualiﬁcation>

where <y1> is a variable containing the number of defective items in each sub-group;

<x> is a variable containing the sub-group identiﬁer (usually 1, 2, 3, ...);

and where the <SUBSET/EXCEPT/FOR qualiﬁcation> is optional.

EXAMPLES

C CONTROL CHART Y X

C CONTROL CHART D X SUBSET X > 2

NOTE 1

The distribution of the number of defective items is assumed to be Poisson. This assumption is the basis for the calculating the upper

and lower control limits.

NOTE 2

The U CONTROL CHART is similar to the C CONTROL chart. The distinction is that the C CONTROL CHART is used when the

material being measured is constant in area and the sub-groups ha ve equal size. The U CONTROL CHART is used when either of these

assumptions is not valid.

NOTE 3

The attributes of the 4 traces that make up the C control chart are controlled by the standard LINES, CHARACTERS, SPIKES, and

BAR commands. Trace 1 is the response variable, trace 2 is the mean line, and traces 3 and 4 are the upper and lower control limits.

Some analysts prefer to draw the response variable as a character or a spike rather than a connected line. The example program

demonstrates setting the line attributes (the control lines are drawn as dotted lines).

DEFAULT

None

SYNONYMS

C CHART for C CONTROL CHART

RELATED COMMANDS

U CHART = Generates a U control chart.

L

CL c 3

c–=

UCL c 3

c+=

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## Sample C Control Chart PDF

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