Process z

Describes the capability of a binary process – a process in which products are judged to be either defective or not defective; some practitioners call Process Z the sigma capability of a process. Process Z is the point on a standard normal distribution such that the area to the right of that point is equal to the Average P (the proportion of defective units in your process). The higher the process Z, the better the process performance; ideally, you want a process Z of 2 or more.

For example, a company manufactures hydraulic pistons and inspects them for cracks. Pistons with cracks are labeled defective and scrapped. Inspectors examine 10 subgroups of pistons and observe the average percent of defective pistons equals 22.64%. This percentage corresponds to an Average P of 0.2264. The point on the standard normal curve with an area of 0.2264 to its right is 0.7507. This value is the Process Z, and represents a poorly performing process. This company needs to improve its manufacturing process.