Understanding Lean Six Sigma Control Charts

Lean and Six Sigma both offer a wealth of process improvement methodologies that help businesses around the world to minimise their waste and produce greater outputs. Control Charts are a primary Lean Six Sigma control technique. They are frequently used to establish controls over a process, monitor project results and measure the outcomes being achieved. Based on simple principles, control charts give practitioners an easy tool to collect and visualise process data in a way that is easy to communicate to an organisation’s key stakeholders.


What is a Lean Six Sigma Control Chart?

Control charts are a graphical representation of process behaviour over time and are one of Lean Six Sigma’s primary control techniques. Along the X-axis, control charts show process outcomes as a variation from mean over time, allowing Lean Six Sigma teams to identify undesirable variation. Control charts are often developed prior to improvement efforts as a means to determine whether a process is stable enough to be altered.

Using control charts, Lean Six Sigma practitioners can determine whether a process is in control, monitor changes due to improvement efforts and identify two key types of process variation:

  1. Common-Cause Variation. Common cause variations are inherent in the process and are typically due to chance. For instance, if a control chart were used to track cross-town deliveries, the level of traffic on the road could slow down travel and result in common cause variations in delivery times.
  2. Special-Cause Variation. Special cause variations fall beyond the expected results and are not due to chance. The causes of special variations can often be identified and eliminated. For instance, if a broken down van caused a significant delivery delay, we could identify a need to invest in preventative vehicle maintenance to reduce future occurrences.


Designing a Lean Six Sigma Control Chart

As with any process improvement project, the key to creating effective control charts is to determine what data needs to be measured. Establishing a central measurement allows Lean Six Sigma practitioners to inspect a process and find out whether any improvement is required, as well as measure the effectiveness of any projects that do take place.

The exact metric being measured depends on the process. Continuing our example from above, tracking delivery times is a simple way of determining whether customers received their orders in an acceptable amount of time. For more complex processes, a control chart could measure metrics such as manufacturing time, customer satisfaction, wastage or other relevant process inputs or outputs.


The Components of a Control Chart

Once a process’ critical metrics are determined, control charts can be created to measure and monitor the outcome of any Lean Six Sigma projects. Control charts contain three simple elements that allow practitioners to plot a process over time:

  • A centreline. The centreline is simply calculated as the mean of all input data points.
  • Upper control limit. The upper control limit (UCL) is placed three standard deviations above the centreline.
  • Lower control limit. The lower control limit (LCL) is placed three standard deviations below the centreline.

Control charts rely on probability to determine whether process variations are within expected limits. The typical placement of the UCL and LCL at three standard deviations captures approximately 99.7% of all data in a standard distribution. If a result falls within three standard deviations, it is labelled a common-cause variation. Results that fall outside the control limits are deemed special-cause variations and are likely to be the subject of process improvement.


How Control Charts are Used

Control charts are used throughout process improvement, from identifying unstable processes and tracking improvements to measuring the overall outcome. Lean Six Sigma control charts may be used to:

  • Track the performance of a process to understand whether improvement is required
  • Track the performance of a process to establish controls
  • Provide a simple representation that can be used to discuss the performance of a process or Lean Six Sigma project
  • Reduce the need to manually inspect processes
  • Predict process trends, capacity and performance
  • Determine whether improvement projects are having an effect on the process
  • Capture data that can be used for review and follow-on improvement projects


Ready to Learn How to Create Control Charts? Contact Thornley Group Today

Control charts are among the most robust and ubiquitous tools in use by modern Lean Six Sigma practitioners. Established properly, a control chart is a simple way to collect and realise critical information. This means control charts are one of the most effective tools for demonstrating the value of Lean Six Sigma to organisational leaders and stakeholders. If you would like to learn to create and interpret control charts, book a training course with Thornley Group today. Our instructors offer a range of programmes to suit everyone from individual professionals to corporate groups. Speak to our consultants today if you would like to book your next Lean Six Sigma training course.

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