Inspection is the process of checking the conformance of a final product to its specifications. In most cases, 100% inspection of aprocess is too costly. Therefore, there are established methods of sampling a product or process to characterize its correspondence to its specifications. Statistical methods are used to extract the significant information from large amounts of numerical information. The systematic method of detecting assignable variability in a process is known as statistical process control (SPC). One of the principal tools used in SPC is the control chart. Control charts are a plot of a quality characteristic (such as the average diameter of a sample of parts or the number of defects in a sample) with respect to time. The quality characteristic is compared to control limits. The basic purpose for the chart is to determine whether the quality characteristic is varying within acceptable limits for natural variability or whether the process is out of control. Process capability analysis is a method of determining or assuring that a process can meet specifications. Acceptance sampling is used to determine if parts that have already been produced are acceptable.
By Gary Conner
By George N Bullen FSME