There was an extensive black market (illegal market) for many consumer products in the united states during world war ii. a likely explanation of the black market is that:

Statistical process control charts: a. indicate to the operator the true quality of material leaving the process. b. display upper and lower limits for process variables or attributes and signal when a process is no longer in control. c. indicate to the process operator the average outgoing quality of each lot. d. display the measurements on every item being produced. e. are a graphic way of classifying problems by their level of importance, often referred to as the 80-20 rule.