The difference between Statistical Process Control (SPC) and Statistical Quality Control (SQC)

When it comes to maintaining high-quality standards in manufacturing and other business processes, tools like Statistical Process Control (SPC) and Statistical Quality Control (SQC) are essential.
Although these terms are often used interchangeably, they actually offer different methods and applications for quality control.
In this guide, we’ll break down the key differences between SPC and SQC.

Understanding Statistical Process Control (SPC)

Statistical Process Control (SPC) is a method used to monitor and control a process to ensure that it operates at its full potential.
Its primary focus is on identifying and reducing variations in the process.

How SPC Works

SPC involves using statistical tools and techniques to observe the performance of a process over time.
The key tool in SPC is the control chart, also known as a Shewhart chart.
This chart helps identify points that signal a problem within the process, such as:

-Outliers or data points that fall outside the control limits.

-A run of seven points on one side of the mean, indicating a shift in the process.

-Trends that suggest a consistent increase or decrease in values.

By detecting these signals, businesses can intervene to correct any issues before they result in defective products or delays.

Benefits of SPC

SPC offers several advantages:

-Improved process stability by identifying sources of variation.

-Early detection of potential issues, allowing for proactive measures.

-Better overall product quality and consistency.

Understanding Statistical Quality Control (SQC)

Statistical Quality Control (SQC) is a broader term that encompasses various statistical techniques used to ensure the quality of products and services.
While SPC is a subset of SQC, SQC includes more comprehensive methods and tools.

How SQC Works

SQC employs a variety of statistical tools to inspect, test, and evaluate product quality.
These tools include but are not limited to:

-Descriptive statistics: Mean, median, mode, and standard deviation to summarize data.

-Inspection units: Sampling and testing finished products to ensure they meet quality standards.

-Process capability analysis: Assessing if a process can produce outputs within specified limits consistently.

-Design experiments: Exploring the relationships between factors affecting a process and the output quality.

Benefits of SQC

The use of SQC can lead to multiple benefits:

-Higher product reliability and customer satisfaction due to stringent quality checks.

-Better understanding and control over manufacturing processes.

-Reduced costs associated with rework, scrap, and warranty claims.

Key Differences Between SPC and SQC

While both SPC and SQC aim to improve the quality and reliability of products and processes, there are some notable differences.

Scope and Focus

SPC is narrowly focused on monitoring process performance and ensuring it stays within control limits.
It primarily deals with detecting and reducing variability within a given process.
SPC is usually applied during the production or operational phase.

On the other hand, SQC has a broader scope and is concerned with both process and product quality.
It includes various statistical methods and can be applied during different stages such as design, production, and post-production.

Application Area

SPC is typically used in manufacturing settings where the aim is to maintain a stable production process.
It is highly effective in continuous production processes such as chemical manufacturing or assembly lines.

SQC, however, finds applications in a wider range of industries including healthcare, software development, and service industries.
For example, it may be used to ensure that a software product meets customer requirements or that a health service consistently meets regulatory standards.

Tools and Techniques

The tools and techniques used in SPC are primarily focused on controlling processes.
Control charts, run charts, and cause-and-effect diagrams are some of the main tools used in SPC.

SQC, meanwhile, employs a wider range of statistical methods including Six Sigma tools, regression analysis, and hypothesis testing in addition to the tools used in SPC.

When to Use SPC or SQC

Choosing between SPC and SQC depends on several factors such as the specific objectives, the type of industry, and the stage of the process or product lifecycle.

Use SPC When:

-You need to monitor ongoing production processes.

-Your primary goal is to identify and reduce process variability.

-You aim to maintain consistent process performance over time.

Use SQC When:

-You require a comprehensive approach to quality management.

-You need to evaluate product quality as well as process performance.

-You aim to apply statistical methods across different stages of product development.

Conclusion

Both Statistical Process Control (SPC) and Statistical Quality Control (SQC) are invaluable tools for achieving high-quality standards in various industries.
While SPC focuses on process monitoring and variability reduction, SQC offers a broader range of statistical techniques to ensure overall product and service quality.
By understanding the differences between these two methodologies, businesses can better choose the right tools to meet their specific quality control needs.
Whether you are in manufacturing, healthcare, or any other industry, incorporating SPC and SQC into your quality management systems can lead to significant improvements in efficiency, reliability, and customer satisfaction.