The Four Types of Quality Control Explained

The four widely used types of quality control are process control, acceptance sampling, statistical process control with control charts, and inspection/testing. In practice, organizations blend these approaches to prevent defects, monitor processes, and verify that products or services meet specifications, often under frameworks such as ISO 9001 or industry-specific standards.

At a Glance: The Four Types of Quality Control

Quality control methods can be grouped into four complementary categories that span prevention, monitoring, and verification. The following list outlines each type and its core purpose in a quality system.

1. Process Control: Keeps production or service processes stable and capable through defined methods, controls, and preventive actions.
2. Acceptance Sampling: Uses statistical sampling to decide whether to accept or reject a lot or batch without inspecting every unit.
3. Control Charts (Statistical Process Control, SPC): Monitors process variation over time to distinguish common vs. special causes and trigger corrective action.
4. Inspection and Testing: Verifies conformance of materials, in-process work, and finished goods via measurements and tests.

Together, these methods reduce variation at its source, detect emerging issues early, and confirm that outputs meet requirements before release.

1) Process Control

Process control focuses on designing and running operations so outputs consistently meet requirements. It emphasizes prevention—controlling inputs, methods, environment, and equipment—so defects are less likely to occur.

Key elements and tools

The items below illustrate common mechanisms organizations use to execute process control effectively.

• Standard Operating Procedures (SOPs), work instructions, and checklists
• Equipment calibration and preventive maintenance
• Process capability studies (Cp, Cpk) and parameter limits
• Poka‑yoke (mistake-proofing) and automated controls/interlocks
• Training and certification for critical tasks
• Change control and documented trials for process adjustments

When robust process control is in place, variability shrinks and downstream inspection finds fewer issues, lowering cost of quality.

When it’s best used

Process control is essential in any repeatable operation—manufacturing lines, labs, software release pipelines, clinical procedures, or food service—where consistent execution drives consistent results.

2) Acceptance Sampling

Acceptance sampling determines whether to accept or reject a lot based on a statistically selected sample rather than inspecting 100% of units. It balances inspection effort with risk to suppliers and customers.

How it works

The points below summarize how acceptance sampling plans are chosen and applied in real settings.

• Define Acceptance Quality Limit (AQL) and risks (producer’s and consumer’s risk).
• Select a standard (e.g., ANSI/ASQ Z1.4 or ISO 2859 for attributes; Z1.9 or ISO 3951 for variables).
• Determine sample size and acceptance/rejection numbers for each lot size and inspection level.
• Inspect the sample; accept or reject the lot per the plan’s criteria.
• Use switching rules (normal, tightened, reduced) based on quality history.

Used properly, acceptance sampling controls incoming or outgoing quality with predictable risk while avoiding the cost of 100% inspection.

When it’s best used

It’s effective for incoming material verification, supplier quality management, or finished-goods release when full inspection is impractical, destructive, or too costly.

3) Control Charts (Statistical Process Control, SPC)

Control charts are time-ordered graphs that show process behavior relative to statistically derived control limits, typically near ±3 standard deviations from the process mean. They detect special-cause variation so teams can intervene before defects accumulate.

Common chart types and uses

Different charts match data types and sampling strategies; the list below maps common charts to typical scenarios.

• X‑bar & R or X‑bar & S charts: For subgrouped continuous measurements (e.g., part dimensions, fill weights).
• Individuals & Moving Range (I‑MR): For single observations when subgroups aren’t practical.
• p or np charts: For proportions or counts of defectives in a sample (attributes data).
• c or u charts: For defect counts per unit or per area/opportunity.

Choosing the right chart ensures signals are meaningful, minimizing false alarms while catching real shifts in process performance.

Key practices

The following practices help organizations get maximum value from SPC.

• Separate control limits from specification limits; one shows process stability, the other shows requirement conformance.
• Investigate out-of-control signals (points beyond limits or non-random patterns).
• Act on root causes and verify effectiveness; update control plans accordingly.
• Recalculate control limits only after stable conditions are restored.

Consistent SPC use enables proactive control, reducing scrap, rework, and customer complaints.

4) Inspection and Testing

Inspection and testing verify that materials, components, and finished products meet defined criteria. It includes visual checks, measurements, functional tests, and non-destructive or destructive testing.

Typical stages of inspection

Organizations often structure inspection across the product lifecycle as outlined below.

• Incoming inspection: Verify supplier materials meet requirements before use.
• In‑process inspection: Check critical features during production to catch issues early.
• Final inspection/pre‑shipment: Confirm conformance before release or dispatch.
• Audit sampling: Periodically assess overall conformance beyond routine checks.

This staged approach reduces the chance of passing defects downstream and supports traceability for quality records.

Tools and methods

The tools below illustrate how inspection and testing achieve reliable, repeatable results.

• Gauges, CMMs, and calibrated measurement systems (with MSA studies)
• Functional and environmental tests (e.g., burn-in, vibration, thermal cycling)
• NDT methods (e.g., ultrasonic, X‑ray, dye penetrant, magnetic particle)
• Checklists, go/no-go gauges, and visual standards

Well-designed inspection and testing strengthen confidence in conformance and provide data to improve upstream controls.

How These Four Types Work Together

High-performing quality systems use all four types in a coordinated way: prevent, monitor, sample, and verify. The sequence below shows a typical integration across operations.

1. Design robust processes and controls (process control) aligned to requirements.
2. Monitor stability and capability in real time (SPC/control charts).
3. Apply risk-based sampling where full inspection isn’t feasible (acceptance sampling).
4. Verify conformance at key stages and before release (inspection/testing).

This layered approach reduces risk at the source, detects issues early, and confirms that only conforming output reaches customers.

Avoiding Confusion: Alternate “Four Types” You May See

Some sources describe “four types” as inspection timing rather than QC methods—commonly pre-production, in-process, pre-shipment, and container loading inspections. Those are inspection stages within the Inspection and Testing category above, not separate, comprehensive QC methods.

Practical Tips for Choosing and Applying the Four Types

Organizations can tailor their mix of quality control methods using the guidance below.

• Start with process control: Define critical-to-quality characteristics and stabilize inputs.
• Instrument the process with SPC where variation matters most to customers.
• Use acceptance sampling to manage supplier or outbound risks cost-effectively.
• Right-size inspection: invest in measurement systems and clear criteria; avoid 100% inspection unless safety-critical.
• Continuously feed findings from inspection and SPC into corrective and preventive actions (CAPA).

Aligning methods to risk, cost, and customer impact ensures quality control remains effective and efficient over time.

Summary

The four types of quality control are Process Control, Acceptance Sampling, Control Charts (SPC), and Inspection/Testing. Together, they prevent defects at the source, statistically monitor process stability, balance verification cost with risk, and confirm conformance before release—forming a complete, layered defense that improves reliability, reduces waste, and protects customers.

What are the 4 steps of quality control?

When broken down, quality control management can be segmented into four key components to be effective: quality planning, quality control, quality assurance, and quality improvement.

What are the 4 types of quality inspection?

The four types of quality inspections are Pre-Production Inspection (PPI), During Production Inspection (DPI), Pre-Shipment Inspection (PSI), and Container Loading Inspection. These inspections are performed at different stages of the product lifecycle to ensure quality standards are met, with PPI checking materials, DPI monitoring the production process, PSI verifying the completed product before shipment, and the container loading inspection ensuring proper handling during shipping.
 
Here’s a closer look at each type of inspection:

1. Pre-Production Inspection (PPI)
   • Purpose: To ensure that raw materials, components, and the overall production setup meet quality requirements before manufacturing begins. 
   • When it occurs: Before production starts, often when materials are received from suppliers. 
   • What it checks: Supplier qualifications, raw materials, packaging, and the factory’s production process. 
2. During Production Inspection (DPI)
   • Purpose: To monitor the quality of products as they are being manufactured, identifying any deviations from standards early on. 
   • When it occurs: At various stages during the manufacturing process. 
   • What it checks: The production workflow, materials, and the quality of work-in-progress to reduce cycle times and increase capacity. 
3. Pre-Shipment Inspection (PSI)
   • Purpose: To verify that the final, completed product meets all quality criteria and specifications before it leaves the factory. 
   • When it occurs: After production is complete but before the product is shipped to its destination. 
   • What it checks: Product quantity, packaging, labeling, functionality, and overall condition. 
4. Container Loading Inspection
   • Purpose: To ensure that products are handled professionally and loaded into containers correctly, safeguarding them during the shipping process. 
   • When it occurs: During the loading of products into shipping containers for transport. 
   • What it checks: The container’s cleanliness, the proper placement of goods to prevent damage, and the quantity of products loaded. 

What are the four M’s of quality control?

By implementing best practices that address the 4Ms of manufacturing: Machines, Manpower, Methods, and Material.

What are the four types of quality control?

Quality control can be broken down into four distinct types: control charts, process control, acceptance sampling, and product quality control. A control chart is beneficial in examining changing processes over an extended period of time.