ABSTRACT
It is often inconvenient, impractical, or impossible to take numerical measurements of the type necessary to set up variables control charts. In these cases, the quality characteristic of a unit is judged, or classified, as either conforming or non-conforming based on whether or not it has certain attributes (leaks/does not leak, works/does not work, etc.). Another means of judging the item is to count the number of non-conformities that appear on the unit (number of scratches, dents, holes, etc.). These types of quality characteristics are referred to as attributes. Attribute data can have only two values, such as pass/fail, conforming/non-conforming, present/absent, etc. Even though attribute data cannot be measured, they can be counted. There are four special control charts for analyzing attribute data:
p chart — Plots the fraction non-conforming per sample np chart — Plots the number non-conforming per sample c chart — Plots the number of non-conformities per inspection unit u chart — Plots the average number of non-conformities per
inspection unit
The following are several basic terms that are used in connection with attributes charts. It is important to understand them.
