Using Control Charts

Control charts display standard and nonstandard statistical process control (SPC) control charts. The default control charts provided with the installation of Quality are the industrial-standard charts that are documented in most Quality references, and are categorized by variable and attribute type.

This table shows the use of variable data-control charts:

Equation

Statistic
gv_7bac_sqsy7e71 gv_7bab_sqsy7e6f gv_7baa_sqsy7e6d gv_7ba9_sqsy7e6b gv_7ba8_sqsy7e69 gv_7ba7_sqsy7e67

where:

gv_7ba6_sqsy7e65 gv_7ba5_sqsy7e63

The

gv_7b95_sqsy7e61

/R chart, or average and range chart, is the most commonly used chart in SPC. Each point (k) on the

gv_7b95_sqsy7e5f

chart consists of an average

gv_7b95_sqsy7e5d

of individual

gv_7b86_sqsy7e5b

measurements within a subgroup of a constant sample size (n). The R chart represents the range (R), which is the maximum, individual measurement in the subgroup, minus the minimum.
gv_7ba0_sqsy7e59 gv_7b9f_sqsy7e57 gv_7b9e_sqsy7e55 gv_7b9d_sqsy7e53 gv_7b9c_sqsy7e51 gv_7b9b_sqsy7e4f

where:

gv_7b9a_sqsy7e4d gv_7b99_sqsy7e4b

The

gv_7b95_sqsy7e49

/s chart, or the average and standard deviation chart, is a variation of the

gv_7b97_sqsy7e47

chart typically used with subgroup sizes of greater than seven. Each point

gv_7b96_sqsy7e45

on the

gv_7b95_sqsy7e43

chart consists of an average

gv_7b94_sqsy7e41

of individual

gv_7b93_sqsy7e3f

measurements within a subgroup of a constant sample size

gv_7b92_sqsy7e3d

The s chart represents the standard deviation

gv_7b91_sqsy7e3b

.within the subgroup of individual measurement.
gv_7b90_sqsy7e39 gv_7b8f_sqsy7e37 gv_7b8e_sqsy7e35 gv_7b8d_sqsy7e33 gv_7b8c_sqsy7e31 gv_7b8b_sqsy7e2f

where:

gv_7b8a_sqsy7e2d gv_7b89_sqsy7e2b

The X/MR chart, or individual and moving range chart, is typically used when measurements can't easily be formed into subgroups. Measurements may be expensive or destructive, or time periods between samples may be excessive. Each point

gv_7b91_sqsy7e3b

on the

gv_7b87_sqsy7e27

chart consists of an individual

Individual measurement

measurement. The MR chart represents the difference between the previous point and the current point (absolute value).

This table shows the use of attribute data-control charts:

Equation

Statistic
gv_7b87_sqsy7e27 gv_7b84_sqsy7e21 gv_7b83_sqsy7e1f

where:

gv_7b82_sqsy7e1d gv_7b81_sqsy7e1b

The p chart displays the proportion of nonconforming items in a group of items being inspected. The plotted point (p) is the fraction of defective items found for each sample (n). The sample size (n) need not be constant; however, sample sizes that vary more than 25 percent may provide misleading results (as documented in most SPC references).
gv_7b80_sqsy7e19 gv_7b7f_sqsy7e17 gv_7b7e_sqsy7e15

where:

gv_7b7d_sqsy7e13

The np chart displays the nonconforming items in a group of items being inspected. It is similar to the p chart, but requires a constant sample size (n). The plotted point (np) is the number of defective items found for each sample (n).
gv_7b7c_sqsy7e11 gv_7b7b_sqsy7e0f gv_7b7a_sqsy7e0d

where:

gv_7b79_sqsy7e0b

The c chart displays the number of defects found in a group of items being inspected. It requires a constant sample size (n). The plotted point (c) is the number of defects found for each sample (n).
gv_7b78_sqsy7e09 gv_7b77_sqsy7e07 gv_7b76_sqsy7e05

where:

gv_7b75_sqsy7e03

The u chart displays the number of defects found in a unit. Each unit is equal to the sample size, which may vary from group to group. This chart is similar to the c chart, but doesn't require a constant sample size (n). The plotted point (u) is the number of defects per unit (sometimes denoted as DPU).