SPSS® 12.0 Command Syntax Reference

Table 4 Each case is a subgroup for p and np charts
Subgroup Relapse N
January 1 4
February 3 4
... ... ...
Variable Specification
SPCHART 1515
If data are organized as illustrated in Table 3, the variable specification on P or NP subcommands
is
VAR BY VAR
The variable specified before BY is the status variable (for example, OUTCOME in Table 3). The
value of this variable determines whether an item is considered conforming or nonconforming.
The status variable can be any type, but if it is a string, the value specified on CONFORM (or
NONCONFORM) must be enclosed in apostrophes (see “CONFORM and NONCONFORM
Subcommands” on p. 1521). The variable specified after BY is the category variable. It can be
any type of variable. The chart is sorted by values of the category variable.
If data are organized as shown in Table 4, the variable specification on P or NP is
COUNT(VAR) N({VAR}) [BY VAR]
{VAL}
The variable specified on keyword COUNT is the variable containing the number of nonconforming
or defective items (for example, RELAPSE in Table 4). The specification on keyword
N is either the variable containing the sample size or a positive integer for a constant size
across samples (for example, N in Table 4). The COUNT variable cannot be larger than the N
variable for any given subgroup; if it is, the subgroup is dropped from calculation and plotting.
Keyword BY and the category variable are optional. When specified, the category variable
is used for category axis labels; otherwise, the category axis is labeled 1 to the number
of subgroups. Cases are unsorted for the control chart.
C and U Subcommands
C produces a c chart and U produces a u chart. Both types are control charts for attributes.
That is, they use data that can be counted.
A c chart plots the total number of defects or nonconformities in each subgroup. A defect or
nonconformity is one specification that an item fails to satisfy. Each nonconforming item has
at least one defect, but any nonconforming item may have more than one defect. The center line
of the c chart indicates the mean of the defect numbers of all subgroups. The control limits are