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What’s the Difference Between SPC and SQC?

Posted on March 19, 2021 by Tiffany M. Donica
Tiffany M. Donica


Both SPC and SQC help to drive smooth operations to promote efficient output and optimal results. They both play a role in overall success in operations, but those two roles are different. Here’s what to know about SPC and SQC to determine which is right for your Food & Beverage or CPG manufacturing facility.

Statistical Quality Control vs. Statistical Process Control

The American Society for Quality defines statistical quality control (SQC) as the application of statistical and analytical tools to monitor process outputs. Statistical process control (SPC), on the other hand, is the application of the same tools to control process inputs.

In other words, SPC encompasses activities that monitor processes in real-time to prevent defects. With SQC, the goal is to ensure that each output, or product, meets customer specifications, and activities such as additional inspection typically occur after manufacturing has already taken place. There’s room for both SPC and SQC in a facility, but what’s most important is looking at the right parameters at the right intervals. Technology such as Plant Management Software is now available to measure SPC and SQC, thus closing the gaps between when a problem occurs and when it’s addressed.  

Below is a closer look at what SPC entails.

Statistical Process Control Principles

SPC leverages statistical methods and sampling programs to help plant managers understand and control variability in manufacturing. With the help of Plant Management Software, process variations are displayed in real-time charts. When a process deviates from standard limits, it sets off an alert. This helps manufacturers better manage their lines and reduce rework and waste. The notifications allow them to step in and address an issue promptly.

Process Control Tools & Techniques

There are a few different SPC methods used to improve yield and drive profits. One important tool is control limits. These are the set specifications within which processes are expected to run. When a process exceeds these limits, alerts can be triggered well before actual specification limits are approached. SPC also uses run rules to monitor processes, as well as process capability measures and histograms.

Here are a few specific SPC tools to consider.

  • SPC Control Charts: These real-time monitors show how a process changes over time via periodic samples. Xbar or mean charts are used to calculate the average values of a sample set, while Xbar-R (range) charts display sample sizes of two to nine data points. An Xbar-S (sigma) chart is used for more than ten data points.
  • SPC Control Limits & Run Rules: Control limits may be used alongside control charts to show a process’s anticipated upper and lower boundaries. These limits are illustrated by three sigmas, forming the six-sigma range with the target in the middle. Run Rules determine when a process has drifted and is no longer stable.
  • Histograms: These charts look at control and can be used to help manufacturers meet customer specifications. They’re used to analyze process variation. Manufacturers account for some process variation and use tools to determine how much a process varies. The objective is to have limited variation, however, as illustrated by a narrow bell curve with shorter tails, which suggests more values fall within specification limits.
  • CPK & PPK (Process Capability Measures): Process capability measures indicate how well your process is conforming to specifications. CPK allows you to look at the variability of sample populations within standard deviations, whereas PPK looks at normal process capability.

Why SPC Is Important

When powered by sophisticated Plant Management Software, SPC allows you to identify issues as they occur to correct them quickly. It also lets you determine if a particular process is considered stable, and gives you the insights needed to figure out if variations in processes are inherent or caused by special circumstances. In other words, findings from your SPC data can tell you whether you need to prevent isolated problems or rework processes completely to drive better quality outcomes


Statistical Quality Control Methods

Again, statistical quality control is the application of statistical and analytical tools to monitor the outcomes of your processes. While SPC measures independent variables, SQC monitors dependent variables. One popular method is acceptance sampling, which is used when a decision needs to be made to accept or reject finished products. For instance, if a number of defective products are low in a batch, the lot may be accepted, but an entire lot may be rejected or reworked if the sample set shows a high incidence of poor-quality products. SQC can use the same tools as SPC, but looks at the outputs instead of inputs.

Statistical Methods for Quality Assurance

There are seven quality control tools which are identified in the Guide to Quality Control, written by Dr. Kaoru Ishikawa. In addition to the histogram and control charts, described above, these include:

  • Cause-and-effect or fishbone diagram: This simple yet effective graph maps out the possible causes of an issue.
  • Pareto chart: This bar graph helps to identify which different scenarios would be most significant, with bar lengths indicating cost or frequency.
  • Check sheet: A simple way to track an event and its causes, this graph allows managers to spot patterns, such as issues occurring during one particular shift.
  • Scatter diagram: These graphs, also called X-Y graphs, are useful for seeing whether there’s a relationship between two sets of numerical data.
  • Stratification: This tool takes data and sets it into specific groups. You might use it to look at shifts, materials, or equipment, for example, assigning one color to each set. This can help illustrate patterns that could reveal potential root causes of issues.

Why SQC Is Important

Whereas SPC helps companies reduce variation in processes and run efficiently, SQC allows manufacturers to meet exact customer requirements. Thus, in Food and Beverage or CPG manufacturing facilities, where there may be many different products being produced, SPC can verify that operations are running efficiently overall whereas SQC can ensure each product is meeting customer specifications.

In Conclusion: The Difference Between SPC & SQC

While the differences between SPC and SQC are clear, there continues to be some debate about whether the two terms can be used interchangeably. This is likely due to the fact that, simply put, both aim to improve your operations and their outcomes. The two are inextricably linked; refining your processes will typically lead to improvements in quality while addressing quality issues often means going back and enhancing your processes.

Whether your plant emphasizes SPC or SQC or marries the two, what matters most is that you’re not simply collecting statistics, but doing so consistently and using your findings to drive continuous improvement.


About SafetyChain Software

SafetyChain is the #1 Plant Management Platform that improves yield, maximized productivity, and ensures compliance for food, beverage, & CPG manufacturers. Trusted by over 1,500 facilities, SafetyChain is the only complete solution for production (OEE & SPC), food safety and quality (QMS), and supplier compliance.

Topics: SPC