Question

In general, What condition must exist prior to calculating process capability? Discuss how process capability can be estimated through control charts.

Answer 1

Before calculating process capability four ‘basic’ conditions have to be met:

1. The process is unadjusted.

2. Samples are gathered in the order of production.

3. The samples comprise rational subgroups.

4. The process is in a state of ‘statistical control.’

Once you’ve met these conditions you can measure the natural, current process capability.’

(If the process has more than one response of the same characteristic, and they’re not random or independent; you’ll have to study the process capability of the process mean and the process uniformity. This approach assumes each response is independent. If it is not; you’ll have to use miltivaraite methods.)

According to Shewhart, the goal of a process control chart is to find sources of variation and, if possible, to remove them.

The ability of a process to meet specifications can be expressed as a single number using a process capability index or it can be assessed using control charts. Either case requires running the process to obtain enough measurable output so that engineering is confident that the process is stable and so that the process mean and variability can be reliably estimated. Statistical process controldefines techniques to properly differentiate between stable processes, processes that are drifting (experiencing a long-term change in the mean of the output), and processes that are growing more variable. Process capability indices are only meaningful for processes that are stable (in a state of statistical control).