preventive maintenance. Operator errors, on the other hand may be caused by a lack of training or experience. In other cases, the characteristics of the process may be at fault.
Histograms show the distribution of a performance measurement such as the number of shortages, or the number of defects, over a period of time. Figure 87 illustrates the number of material shortages per day for a particular period. The chart reveals that the number of shortages ranges from one to fifteen per day with eight representing the most frequent number. Other types of graphs include pie charts and line graphs. Pie charts are used to show the various parts of a problem, process or financial measurement in proportion to the whole. Figure 8-8 illustrates the various types of product costs in proportion to total factory costs. The idea is to emphasize the areas with the greatest potential for improvement. Line charts are very useful for showing trends in nearly any type of measurement. For example Figure 8-9 illustrates the number of parts shortages over a fifty-two week period. 18