Statistical Process Control (SPC) charts are a valuable tool for assessing performance and progress in a project. These charts provide a visual representation of performance over time, with upper and lower limits indicating the levels of variation. By interpreting SPC charts, you can gain insights into the behaviour of the system.

There are four rules that help identify the system’s behaviour when analyaing an SPC chart. If any of these rules are broken, it indicates the presence of “special cause” variation in the system. However, it is important to note that it is normal for a process to exhibit only “common cause” variation, showing no signs of special cause.

  1. Any point outside the control limits
  2. Seven consecutive points all above or all below the centre line, or all increasing or decreasing:
  3. Unusual pattern or trends within the control limits:
  4. Number of points inside middle third of the region between the control limits differs markedly from two -thirds of the total number of points:

The goal of SPC charts is to reduce variation and create a more efficient system. Common causes and special causes of variation require different types of improvement strategies. Controlled variation (common cause) indicates a stable and predictable process, where the variation is inherent and the system may need to be changed. On the other hand, uncontrolled variation (special cause) suggests an unstable and unpredictable process that requires immediate attention.

When interpreting SPC charts, it is crucial to consider the following:

  • Reacting to special cause variation should not lead to changing the process without understanding the underlying factors.
  • Ignoring special cause variation by assuming it is part of the process can hinder improvement efforts. It is essential to identify and address external factors contributing to the variation.
  • Ensuring that the chart is not comparing multiple processes or displaying false signals is important to maintain accuracy.
  • SPC charts, when used in conjunction with other investigative tools like process mapping, help understand the scale of the problem and identify possible causes. Additionally, they enable the measurement of the impact of improvements on variation.

SPC is a tool that will help you understand the scale of any problem (the degree of variation) and identify possible causes when used with other investigative tools, e.g. process mapping.

Find out more

The Making Data Count Team is part of the Intensive Support for Challenged Systems Team at NHS England. The team works across the NHS to support Trusts and teams in adopting statistical process control (SPC), which supports better decision-making over RAG reporting. They have created SPC tools for organisations to use and we provide a broad range of training all staff including Trust boards, analysts, clinicians and managers in adopting SPC and making their data count!

Need an SPC chart template? Download the latest version from NHS England’s Plot the Dots team.

The THIS institute have a series of guides on the Elements of Improving Quality and Safety in Healthcare published through Cambridge University Press. Recommended guide for this topic is Statistical Process Control, by Mohammed Amin Mohammed (2024).