Defect and defective

a flaw versus an unusable unit

A defect is a single nonconformity, such as a scratch. A defective unit is a unit with at least one defect. One unit can have several defects.

This difference decides the chart: p and np charts count defective units, c and u charts count defects.

Calculate it

Attribute Control Charts: p, np, c and u charts for defectives and defects, including variable sample sizes.