What is the best way to interpret statistical process control charts?

1 Types of SPC charts

Depending on the nature of your data and the purpose of your analysis, there are various types of SPC charts. Run charts display the variation of a single variable over time, and are useful to detect shifts, cycles, or outliers in your process. Control charts are run charts with upper and lower control limits that indicate the expected range of variation for a stable process. Histograms show the frequency distribution of a variable and can help assess the shape, spread, and central tendency of your data. Pareto charts show the relative importance of different categories or factors that contribute to a problem or an outcome. This type of chart is useful for prioritizing improvement actions based on the 80/20 rule. Finally, scatter plots illustrate the relationship between two variables and can help assess the correlation, direction, and strength of the association between them.

2 Steps to interpret SPC charts

Interpreting SPC charts effectively requires following some basic steps. First, you should define your process and its inputs, outputs, and boundaries to select the appropriate type of chart and relevant data to collect and analyze. Then, choose a sampling method and frequency that will depend on the availability, accuracy, and variability of your data. After plotting your data and calculating the control limits and other statistics, it is time to analyze the chart and look for patterns, trends, or signals. This will help you determine if your process is stable, predictable, and in control or if there are any special causes of variation that need to be investigated. You can also use rules or guidelines such as the seven basic tools of quality or the Western Electric rules to recognize these signals. Finally, take action and monitor the results by eliminating waste, reducing variation, or optimizing parameters. You can also update your chart and compare the before and after data to monitor the impact of your action.

4 Challenges of SPC charts

SPC charts present some challenges, such as collecting and managing data that is accurate, complete, and relevant for your analysis. Additionally, you need to select the right chart for your data and purpose, and apply it correctly and consistently following SPC standards and conventions. Furthermore, you must possess the skills and knowledge to interpret the chart and draw valid conclusions, as well as communicate the results clearly and effectively to your stakeholders.

5 Tips for using SPC charts

To use SPC charts successfully, you can involve your team and customers in the design, implementation, and evaluation of your SPC charts. This will help you gain their input, feedback, and support, as well as improve collaboration and engagement. Additionally, use multiple charts and compare them to gain a more comprehensive and holistic view of your process. Review and update your charts regularly to keep track of progress and identify new opportunities and challenges. Doing so will help you make the most of SPC charts.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?