Looking Beyond Cpk in Manufacturing

Today I’d like to revisit an index that any Six Sigma practitioners would love to talk about: Cpk or Process Capability Index. Rather than explaining Cpk in length, I’ll put the formula straightaway below.

In short, Cpk measures how far away your process is with respect to the upper and lower tolerance limit in terms of standard deviation. The higher Cpk is, the tighter the process. This is why factory managers love to see high Cpk numbers — it suggests that that their processes are good. It is commonly accepted that a Cpk lower than 1.3 (sometimes 2) means that the…