SPC Charts: Attributes

SPC attribute charts provide different perspectives for reviewing defective units. What constitutes a defective unit depends on your definition (e.g. a unit might have to have three defects or certain type of defects  to be classified as defective).

  • Chart: nP
  • Chart: P
  • Chart: C
  • Chart: U
  • Chart: Exponentially Weighted Moving Average (EWMA)
  • Chart: Report

Chart: nP

nP charts:

Track Number of defective units in each sample.
Report Number of defective units in each sample.

Note: Each unit is treated as pass or fail regardless of the number of defects it might have.

nP chart sample is:

  • A constant:size.
  • Normally collected in a frequency that emulates the process being evaluated.

  • The nP chart is selected on the:
  1. Attribute Quality Characteristics Properties dialog box>General tab.
  2. SPC Control Properties dialog box>Chart tab.

Chart: P

P charts:

Track Number of defective units in each sample.
Report Proportion (percent) of defective units in each sample

Note: Each unit is treated as pass or fail regardless of the number of defects it might have.

P chart subgroup is

  • Possibly different from other subgroups in size.
  • Normally collected in a frequency that emulates the process being evaluated.
  • The nP chart is selected on the:
  1. Attribute Quality Characteristics Properties dialog box>General tab.
  2. SPC Control Properties dialog box>Chart tab.
  • Control tests are available for the P chart.

Chart: C

C charts:

Track Total number of defects in a unit.
Report Total number of defects in a unit.

C chart subgroup is:

  • Either:
  • One item.
  • A fixed number of items that are treated as one item.
  • Assigned one or more defects to track.
  • The C chart is selected on the:
  1. Attribute Quality Characteristics Properties dialog box>General tab.
  2. SPC Control Properties dialog box>Chart tab.

Chart: U

U charts:

Track  Total number of defects per unit that occur during the sampling period.
Report Mean (average) defects per unit during the sampling period. Note:Currently in CIMPLICITY SPC, the UCL is 1 or lower.

U chart subgroup is:

  • Able to have a different number of sampling units in each sampling period
  • Assigned one or more defects to track.
  • The U chart is selected on the:
  1. Attribute Quality Characteristics Properties dialog box>General tab.
  2. SPC Control Properties dialog box>Chart tab.

Chart: Exponentially Weighted Moving Average (EWMA)

EWMA charts

Track Number of defective units in an observation.
Report Weighted subgroup mean, the more recent the sample, the higher the weight. Note: This detects much smaller process shifts than a normal control chart would

EWMA observation is a rolling, fixed size weighted subset.

The EWMA chart is selected on the SPC Control Properties dialog box>Chart tab.

Chart: Report

Report charts display variable values in table format.

Optional fields that display on the Report chart are selected on the SPC Control Properties dialog box>Report tab.