Quantitative Analysis for Management, 13/e

15.3 Control Charts for Variables

Control charts for the mean, $\overline{x},$ and the range, R, are used to monitor processes that are measured in continuous units. Examples of these would be weight, height, and volume. The $\overline{x}$-chart (x-bar chart) tells us whether changes have occurred in the central tendency of a process. This might be due to such factors as tool wear, a gradual increase in temperature, a different method used on the second shift, or new and stronger materials. The R-chart values indicate that a gain or loss in uniformity has occurred. Such a change might be due to worn bearings, a loose tool part, an erratic flow of lubricants to a machine, or sloppiness on the part of a machine operator. The two types of charts go hand in hand when monitoring ...