What is Statistical Process Control (SPC) and How Does It Improve Quality?

What is Statistical Process Control?

Statistical Process Control (SPC) uses statistical methods to monitor a manufacturing process and understand how it changes over time.

It helps teams distinguish normal process variation from unusual changes that may require investigation.

In simple terms

Suppose a machine produces metal shafts with a target diameter of 10 mm.

Even when the process is operating normally, measurements will vary slightly.

SPC helps determine whether that variation is expected or whether something unusual may have happened.

Simple Example

A machine produces shafts with a target diameter of 10 mm.

Measurements remain stable around the target for several hours.

Suddenly, measurements begin increasing consistently.

Even if the parts are still within specification, this trend may indicate tool wear, temperature changes, or machine drift.

A control chart can help identify this pattern before defective parts are produced.

Key SPC Tools

Control Charts Monitor process behavior over time.

Histograms Show the distribution of measurements.

Check Sheets Organize collected data.

Pareto Analysis Identify the most frequent or significant problems.

Important Insight

Control limits and specification limits are not the same.

• Control limits are calculated from process data and help assess process stability.
• Specification limits are defined by customer or design requirements.

A process can be stable but consistently produce parts outside specifications. It can also produce parts within specifications while showing signs of instability.

How to Apply SPC

• Select a critical process characteristic.
• Collect measurements in time order.
• Choose an appropriate control chart.
• Establish control limits using suitable data.
• Monitor for unusual patterns or signals.
• Investigate special causes before making adjustments.
• Verify whether corrective actions improve stability.

Benefits

• Early detection of process changes
• Reduced scrap and rework
• Better process consistency
• Improved quality decisions
• Less dependence on final inspection
• More reliable manufacturing performance

Practical Insight

Don’t wait for a defect report to discover that a process is changing.

SPC helps you detect signals in the process before those signals become quality problems.