Why would a manufacturing company sample 25 parts each hour and use a process control chart to maintain the?

mean diameter of parts produced rather than measure each part produced?Why would a manufacturing company sample 25 parts each hour and use a process control chart to maintain the?
For economic reasons. I pressume that the company is using human inspectors.


100% inspection is not a guarantee of the quality of the finished product. The quality being defined as meeting standards or specifications.


If it costs $1 to inspect one unit and the production per hour is 1000 units that is a savings of $975 dollars per hour.


For example, the sample size is dictated by the lot size and the Average Quality Level (AQL). Normally, majority of the manufacturers use Military Standards, e.g. 105D, hence the term AQL. Statistically speaking, the Mean (X-bar) will represent the population. Using the three-sigma (plus or minus three standard deviations from the mean) control limits the control chart will represent almost 100% of the


population. SPC (using standard deviation) will estimate the errors of a given population by using only a smaller sample size rather than inspecting the whole production or lots.


Refer to Quality Control Handbook, Third Edition, J. M. Juran, et. al., editors, 23-1, Process Control by Statistical Methods. Also Sections, 22, 24, and 25.