3. Statistical Control Charts
a. Statistical control charts are graphic aids for monitoring the status of any process subject to acceptable or unacceptable variations during repeated operations.
1) They also have applications of direct interest to auditors and accountants, for example, (a) unit cost of production, (b) direct labor hours used, (c) ratio actual expenses to budgeted expenses, (d) number of calls by sales personnel or (e) accounts receivable.
b. A control chart consists of three lines plotted on a horizontal time scale.
1) The center line represents the overall mean or average range for the process being controlled. The other two lines are the upper control limit (UCL) and the control limit (LCL).
2) The processes are measured periodically, and the values (X) are plotted on the chart.
a) If the value falls within the control limits, no action is taken.
b) If the value falls outside the limits, the result is abnormal, the process is considered out of control, and an investigation is made for possible corrective action.
c. Another advantage of the chart is that it makes trends and cycles visible.
1) A disadvantage of the chart is that it does not indicate the cause of the variation.