Capability analysis for attributes
Attributes capability measures are taken directly from attributes control charts. No additional calculations are required.
The capability for a p-chart is the average proportion of nonconforming items (p-bar). The capability for an np-chart is the average number of nonconforming items generated by the system (np-bar). The capability for a c-chart is the average number of nonconformities per subgroup (c-bar). The capability for a u-chart is the average number of nonconformities per unit (u-bar).
A weakness in capability estimates for attributes data is that they do not suggest why a system is either capable or not. For instance, there is no way of knowing whether the system is incapable because it is not centered, it is too close to a specification limit , or it exhibits too much unit-to-unit variation . Further studies must be done to learn how to improve the system.