How do you use SPC to monitor and control process variation and performance?

1 Define the process and the requirements

The first step of using SPC is to define the process you want to monitor and control, and the requirements or standards that you want to meet. This includes identifying the input variables, the output variables, the process steps, the equipment, the materials, and the environmental factors that influence the process. You also need to define the quality characteristics or specifications that your product or service must comply with, such as dimensions, weight, color, strength, or functionality.

2 Collect and organize the data

The second step of using SPC is to collect and organize the data that measures the quality characteristics of your process output. You need to decide how often, how much, and how to collect the data, and use appropriate sampling techniques to ensure the data is representative and accurate. You also need to organize the data in a format that is easy to analyze and interpret, such as a spreadsheet, a database, or a software program.

3 Select and construct the control charts

The third step of using SPC is to select and construct the control charts that will display the data and show the variation and performance of your process. Control charts are graphical tools that plot the data over time and compare them with the upper and lower control limits, which are calculated based on the statistical properties of the data. There are different types of control charts for different types of data, such as variable data (e.g., length, weight, temperature) or attribute data (e.g., defects, errors, complaints). You need to choose the appropriate control chart for your data and construct it using the formulas or software available.

4 Analyze the control charts

The fourth step of using SPC is to analyze the control charts and look for patterns, trends, or signals that indicate the variation and performance of your process. You need to distinguish between common cause variation, which is inherent and random in the process, and special cause variation, which is abnormal and assignable to a specific source. You can use various rules or tests to identify the presence of special cause variation, such as points outside the control limits, runs of points above or below the center line, or shifts or cycles in the data. You also need to evaluate the performance of your process by comparing the data with the specifications or targets, and calculating the capability indices, such as Cp, Cpk, Pp, or Ppk.

5 Implement corrective actions

The fifth step of using SPC is to implement corrective actions to eliminate or reduce the special cause variation and improve the performance of your process. You need to investigate the root causes of the variation and identify the potential solutions that can prevent or minimize them. You also need to plan and execute the corrective actions, such as adjusting the equipment, changing the materials, training the operators, or modifying the procedures. You need to document and communicate the corrective actions and their results to the relevant stakeholders.

6 Monitor and review the process

The sixth and final step of using SPC is to monitor and review the process and its variation and performance over time. You need to continue collecting and plotting the data on the control charts and analyzing them for any changes or deviations. You also need to review the effectiveness of the corrective actions and their impact on the process capability and quality. You need to update the control limits, specifications, or targets as needed, and look for opportunities for further improvement or optimization.

7 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?