What is Process Z?
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.
You can use Process Z in Binomial Capability Analysis to estimate the sigma capability for attribute data. Process Z describes the capability of a binary process - a process in which products are judged to be either defective or not defective.
Example of how the average p-value and the Process Z value are related
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.
Standard normal density curve
A process with an average P of 0.2264 has a Process Z of 0.7507.
Display the Process Z value
The process Z values is available in the results of your binomial capability analysis.
Suppose the defects are in column C1 and the subgroup sizes are in column C2 of your worksheet. The process is in statistical control and rational subgroups were collected to reflect the inherent variation of the process.
- Choose .
- In Defectives, enter C1.
- In Use sizes in, enter C2.
- Click OK.
