Quality control procedures is the statistical process designed to ensure that the precision and accuracy of, for instance, a laboratory test, are maintained within the acceptable limits. The simplest such process involves the chart (generally called a control chart) with three horizontal lines, one made at the target level of the applicable control statistic, and the others, called action lines, made at some pre-specified distance above and below the target level. The procedure is examined to be at an acceptable quality level as long as the observed control statistic lies among the two lines and to be at a rejectable quality level if any of these lines are crossed.