Graphical analysis of capability of a process producing a product family

Statistical techniques are effective and powerful means of quantifying the variability of processes, analyzing this variability with reference to product requirements, and eliminating this variability in product manufacturing. Many process capability indices have been effectively and widely used to determine whether the quality of a process meets preset targets. However, conventional process capability indices cannot be applied to assess the entire process capability of a product family with nominal-the-best specifications. This work presents a novel process capability index (C _pp ^T ), which takes into account all family members. The index C _pp is a simple transformation from index C _pm , and C _pp ^T provides additional, individual information concerning the accuracy and precision of a process. Vännman's (δ, γ)-plot [Vännman and Deleryd, Quality and Reliability Engineering International 15(3): 213-217 (1999)] is revised to compare the process capabilities of family members under both 100% inspection and sampling plans. Examples are provided to demonstrate the method's practical application.