A statistical approach to telephone transmission maintenance

Trunk-net-loss deviation, i.e., the difference between measured and expected value of trunk loss, may be considered to be the outcome of a complex process, the result of many variables. In general, a series of such observations forms a statistical distribution. Such distributions have variability, dispersion, or spread. The measure of this is standard deviation, designated as ¿¿. This variability can be analyzed into two components, one due to findable causes, and one due to chance. If action is taken when chance alone is operating, more variability is introduced than was present previously. If action is not taken when findable causes are operating, unnecessary troubles are allowed to vitiate the process. The Shewhart control chart is a convenient device for detecting findable causes on background of chance causes. Its application to the telephone transmission maintenance problem is discussed.