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SPSS® 12.0 Command Syntax Reference

SPSS® 12.0 Command Syntax Reference

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1516 SPCHART<br />

based on the Poisson distribution and can be controlled by the user-specified SIGMA value.<br />

When the sample sizes are not equal, c charts are not recommended.<br />

Figure 8 C chart<br />

A u chart plots the average number of defects or nonconformities per inspection unit within<br />

a subgroup. Each subgroup contains more than one inspection unit. The center line of the u<br />

chart indicates the average number of defects per unit of all subgroups. The control limits are<br />

based on Poisson distribution and can be controlled by the user-specified SIGMA value.<br />

Figure 9 U chart<br />

Data Organization<br />

Data for c and u charts can be organized in two ways: each case is a unit or each case is a<br />

subgroup.<br />

• Each case is a unit with a variable containing the number of defects for that unit and a<br />

subgroup identifier. Cases are assigned to each subgroup by the value of the identifier.<br />

Table 5 is an example of this type of data organization. Data do not have to be sorted by<br />

subgroup. A BY variable (the subgroup identifier) is required to sort and aggregate data<br />

and to label the category axis.<br />

• Each case is a subgroup. One variable contains the total number of units within the<br />

subgroup and one variable contains the total number of defects for all units within the<br />

subgroup. The subgroup identifier is optional. When specified, it is used as category axis<br />

labels; otherwise, the number 1 to the number of subgroups are used to label the category

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