The term control has various meanings, including supervising, governing, regulating, or restraining. The control in quality control means defining the objective of the job, developing and carrying out a plan to meet that objective, and checking to determine if the anticipated results are achieved. If the anticipated results are not achieved, modifications are made in the work procedure to fulfill the plan.

One way to describe the foregoing is with the Deming Cycle (or PDCA circle; see Figure 5.1), named after Deming in Japan because he introduced it there, although it was originated by Shewhart. It was the basis of the turnaround of the Japanese manufacturing industry, in addition to other Deming management principles. The word management describes many different functions, encompassing policy management, human resources management, and safety control, as well as component control and management of materials, equipment, and daily schedules. In this text, the Deming model is applied to software quality.

In the Plan quadrant of the circle, one defines objectives and determines the conditions and methods required to achieve them. It is crucial to clearly describe the goals and policies needed to achieve the objectives at this stage. A specific objective should be documented numerically, if possible. The procedures and conditions for the means and methods to achieve the objectives are described.

In the Do quadrant of the circle, the conditions are created and the necessary training to execute the plan is imparted. It is paramount that everyone thoroughly understands the objectives and the plan. Workers need to be taught the procedures and skills required to fulfill the plan and thoroughly understand the job. The work is then performed according to these procedures.

In the Check quadrant of the circle, one must check to determine whether work is progressing according to the plan and whether the expected results are obtained. The performance of the set procedures must be checked against changes in conditions, or abnormalities that may appear. As often as possible, the results of the work should be compared with the objectives. If a check detects an abnormality—that is, if the actual value differs from the target value—then a search for the cause of the abnormality must be initiated to prevent its recurrence. Sometimes, it is necessary to retrain workers and revise procedures. It is important to make sure these changes are reflected and more fully developed in the next plan.

In the Action quadrant of the circle, if the checkup reveals that the work is not being performed according to plan or results are not what was anticipated, measures must be devised for appropriate action.

Going around the PDCA Circle