Question

How many defectives parts can we expect from a process that is in a state of statistical control (a stable process) ?

A.	2700 PPMs (0.27%)

B.	Zero defectives

C.	We don’t know

D.	5% of production

Answer 1

When a process is in statistical control, we say that the process is maintaining a 4 sigma / 6 sigma/8 sigma limit etc. As per statistical table ( for normal distribution), certain percentage of material will be within such control limits. For example 95.44 percent of items will be within 4 sigma control limits. 99.73% of items will be within 6 sigma control limits. But in all cases, 100 percent of items will not be within control limits. That would imply a certain percentage will be outside control limits. But we cannot quantify since it would depend on spread of sigma as mentioned above.

Based on above logic, the answer therefore should be “C ) We don’t know”

Answer : C) We don’t know