SPC Attributes

An attribute is a visual characteristic that is checked to determine the fitness of a part, including Pass Fail, Defect Count and Tally Sheet.

Attribute Types

Pass Fail

It allows for tracking whether the characteristic has passed or failed, e.g. is the sticker on? Any fail also makes the part defective. It is typically used with go no-go requirements.

Defect Count

Used to track the number of defects on a characteristic, e.g., number of scratches. The user determines when the part is defective.

Tally Sheet

Count the number of defects on a characteristic in random order at any time during the routine based on sample size. The user determines when the part is defective.

Common Attributes

P Chart

The most commonly used attribute control chart is the p chart. The p chart is used specifically to control the fraction or proportion non-conforming. The fraction non-conforming is the ratio or the quantity non-conforming divided by the quantity of parts. The p chart is used for several reasons:

  • On processes that use go no-go gaging such as plug gages, ring gages and functional gages.
  • In inspection areas to monitor or control the fraction non-conforming from the shop or supplier.
  • In situations where visual inspection is performed.
  • Other attribute situations where the fraction (or proportion) non-conforming requires control.
  • Attribute Inspection where sample size varies.

Pareto Chart

The pareto chart displays defect types as a fractional percentage of the entire defect population. This allows you to distinguish the “significant few” from the “trivial many”. The Pareto chart displays columns of fractional percentage as well as plot points of the cumulative percentage.
c chart, u chart, np chart, pie chart and tallysheet are also available.