The ability to achieve breakthrough improvements, meet new standards and consistently maintain high levels of performance depends upon the effectiveness of controls.

"Last but not least," may be a cliche, but in the world of Six Sigma manufacturing, it's an expression that surely holds sway.

Consider the five phases of Six Sigma-Define, Measure, Analyze, Improve and Control. While all are important, a Six Sigma project that omits the last phase, "Control," is likely destined for failure. Indeed, the ability to achieve real breakthrough improvements, meet new standards and consistently maintain high levels of performance depends upon the effectiveness of controls. A failure to install new controls on improvements made by a Six Sigma project team may make it impossible to hold any gains realized. In fact, if controls are not precisely put in place, the probability is high that the original problem or problems that were eliminated will return. To hold the gains made to any part of the production process on the shop floor, Control is required to evaluate the actual performance of a product or process feature, compare its actual performance to the standard that has been set and take action on the difference, if any.

Feedback loop
Quality control takes place by using a feedback loop. A generic form of the feedback loop is shown in Figure 1.

The quality control progression takes the following steps:

1. A sensor is "plugged in" to evaluate the actual quality of the control subject -- the product or process feature in question. The performance may be determined directly by evaluating the process feature, or indirectly by evaluating the product feature.

2. The sensor reports the performance to an umpire.

3. The umpire also receives information that identifies the quality goal or standard.

4. The umpire compares actual performance to the standard. If the difference is too great, the umpire energizes an actuator.

5. The actuator stimulates the process, whether human or technological, that changes the performance, bringing quality into line with the goals.

6. The process responds by restoring conformance.

On the shop floor, control can be carried out by machines such as programmable controllers, which are automated feedback loops. Control also can be carried out by humans performing each of the feedback loop functions.

These controls, or feedback loops, are designed by Six Sigma project teams in collaboration with managers and operators in the shop floor process in which control is to be carried out. Sometimes a team is supported by a member of management, called the project's Champion. The standard sequence of events is for the designers to choose control subjects, establish a measurement system and verify its capability, and set standards of performance. Designers define who or what will measure the actual performance, how actual vs. standard will be interpreted and the specific actions to be taken on the difference.

While designing controls, Six Sigma project teams must choose from a multitude of control subjects.

Dominant variables
A salient feature of Six Sigma methodology comes into play when choosing control subjects. The Improve phase produces a detailed, quantified description of the relationships between process inputs that are vital to desired process outputs. In addition to designing controls for outputs-for example, final inspection-project teams design controls for inputs, a more economical and efficient approach to exercising control on the shop floor than through final inspection.

Operating processes are influenced by many variables, such as input materials, physical facilities, human skills and environmental conditions. But one variable is often more important than all the others combined. Such a variable is said to be the dominant variable.

In choosing control subjects, project teams must consider the dominant variables associated with the process being improved, the process changes or improvements that have been made, and the variables that are likely to cause critical failure.

Following is a list of dominant process variables:

Setup dominant. Some processes are highly stable. Their results can be reproduced over many cycles of operation. The design for control should provide the operating forces with the means for precise setup and the means to validate it before operations begin. A common example is sheet metal stamping or a printing process.

Time-dominant. This process is known to change progressively with time, due to factors such as depletion of consumable supplies, machines heating up or tool wear, for example. A design for control should provide the means for periodic evaluation of the effect of any progressive change and for convenient readjustment.

Component-dominant. The main variable here is the quality of the input materials, subassemblies and components. An example is the assembly of complex electronic or mechanical equipment such as computers. For the short run, it may be necessary to resort to inspection of materials from a supplier. For the long run, the design for control should be directed at supplier relations, including joint planning to upgrade supplier inputs.

Worker-dominant. In these processes, quality depends mainly on the skill possessed by the workers. The design for control should emphasize aptitude testing of workers, training and certification, quality rating of workers and error proofing to reduce worker mistakes.

Information-dominant. These processes are of a job shop nature so there is frequent change in the product to be produced. As a result, the job information changes frequently, as in the case of a service department. The design for control should concentrate on providing an information system that can deliver accurate, up-to-date information on how a job differs from its predecessors.

In addition to considering dominant variables, a second type of Control Subject that project teams must address is changes or improvements that have been made to the process. Because these process features are new to the operating forces, special attention is focused on providing formal controls so the new process is followed and the original problem does not return.

A classic example of failure to follow new controls occurred on an assembly line for factory-built homes. Linoleum is installed at an early stage. If damage occurs to the linoleum downstream at a later stage of construction, much work must be undone, replacement made, and the undone work redone.

A Six Sigma project team solved this problem by establishing a simple control subject -- protection for the linoleum. The protection was a covering that was to be applied by the linoleum installers as the final step of installation. The covering was not to be removed until the house was placed on its foundation.

New workers and supervisors were assigned but not trained in the control process. In this factory example, there were no quality assurance personnel to audit the new controls. The results -- expensive linoleum damage soared and the original problem returned. The damage resulted from the protective covering not being placed as defined due to lack of training and standard operating procedures not being established.

A third type of control subject consists of three variables -- conformance to specifications, predictability and self-control-which are usually inputs that have been discovered by means of Failure Mode and Effect Analysis (FMEA) and are likely to cause critical failures if not correctly controlled.

Six Sigma project teams perform FMEA at several points during a project to ensure that adequate controls exist to prevent the most critical type of failures in a process or product. Occasionally, the teams do not wait until the Control phase for installation if FMEA indicates they are needed but are absent. This could happen in the Measure or Analyze phase, but it is always revisited in the Control phase.

A project team has little choice but to address each of three types of Control if the shop floor is to achieve and maintain Six Sigma levels of production. The team must come up with controls for conformance to specifications, predictability and self-control. Each has its own methodology and set of tools.

Self-control, however, deserves further explanation. If humans are to exercise any of the controls developed by a project team, then those individuals must have at their disposal all elements of the feedback loop (See Figure 1). For persons to be in self-control, project teams, together with management, must provide those people with all means for self-control. Workers who are charged with self-control must:

Know exactly what is expected

  • Product standard
  • Process standard
  • Who does what and who decides what
Know what they are doing compared to the standards
  • Timely feedback

Have the ability to regulate the process

  • Capable process
  • Necessary tools, equipment, materials, maintenance and time
  • Authority to adjust

When even one of the subelements enabling self-control is missing, all of the factors necessary to be successful on the job are not available. All of the steps in the feedback loop cannot be exercised. Many successful Six Sigma projects have foundered when personnel carrying out the new controls were not placed in a state of self-control.

The original problem can then return because the new process simply cannot be controlled. Management has the responsibility and the authority to provide employees with the means for self-control. Six Sigma project teams must get management cooperation to do this.

Control is critical to attaining and maintaining the Six Sigma performance levels that project teams need to effectively use the available tools, including statistical control charts, formal control plans, Standard Operating Procedures, mistake proofing and audit plans for quality assurance to confirm that the controls are being followed.

If a company has no formal quality system, it must design and implement one -- probably starting with ISO and then expanding on that by adopting the Six Sigma discipline. Once the Six Sigma process is in place, careful adherence to all five phases, including the last one-Control-can produce handsome payoffs. By providing effective controls, manufacturers will be better able to predict process performance, and will receive advance warning of impending disasters early enough to take corrective action and get processes back on track. Significant improvements in quality and higher levels of customer satisfaction will be among the measurable benefits.


  • Control is critical to attaining and maintaining Six Sigma performance levels.
  • If controls are not precisely put in place, the probability is high that the original problem will return.
  • Many successful Six Sigma projects have foundered when personnel carrying out the new controls were not placed in a state of self-control.