DISTINCTIVE ISSUES OF STATISTICAL PROCESS IN-CONTROL AND NORMAL PROBABILITY DISTRIBUTION IN QUALITY CONTROL

The significant roles play by normal distribution in statistical process control is considered. The normal distribution has useful properties which are exploited in process control, that is, symmetrical distribution. The input of a process usually has at least one or more measurable characteristics that are used to specify outputs. These can be analyzed statistically, where the output data shows a normal distribution the process can be described by the process mean (average) and the standard deviation. A process needs to be established with appropriate process controls in place. A control chart analysis is used to determine whether the process is “in statistical control”. If the process is not in statistical control then capability has no meaning. Therefore the process capability involves only common cause variation and not special cause variation. With normal distribution, the “tails” can extend well beyond ±3𝜎’s,
but this interval should contain about 99.73% of production output.