Key takeaways
- Quality by design (QbD) = building quality into the product and process upfront.
- Quality by inspection (QbI) = finding and removing defects after they happen.
- QbD has higher upfront cost; QbI has higher recurring cost.
- Most plants drift toward QbI because it requires no design change; pay the recurring cost forever.
- The cost arithmetic almost always favors QbD over the life of the product.
Short answer: Quality by design builds quality into the product and process from the start. Quality by inspection finds and removes defects after they have happened. QbD has higher upfront cost; QbI has higher recurring cost. Most plants drift toward QbI because it requires no design change, but the cost arithmetic over product life almost always favors QbD. See also Quality Hold Process Design.
What QbD does
- Design products that are hard to make wrong.
- Process design that prevents defect modes.
- Poka-yoke (mistake-proofing) at every workstation.
- Process capability (Cpk) targeted at design stage.
- Statistical process control to maintain.
What QbI does
- Inspect at multiple points.
- Detect and remove defects.
- Rework or scrap defective output.
- Customer complaint handling.
- Field failure investigation.
Cost comparison over product life
QbD upfront:
- Design engineering time.
- Process engineering investment.
- Tooling and fixtures.
- Operator training.
- Initial validation.
QbI recurring:
- Inspection labor.
- Scrap and rework cost.
- Inspection equipment.
- Customer returns and complaints.
- Reputational cost of escapes.
QbI cost compounds per unit forever; QbD cost amortizes. Over volume, QbD almost always wins.
Why plants drift toward QbI
- QbI requires no design change. Easier to implement.
- Inspection cost looks like operational expense, design cost looks like capital.
- Quality team's job is inspection; they expand it.
- Customer complaints drive inspection additions, not design changes.
Drift is gradual but persistent.
How QbD works in practice
- Critical-to-quality (CTQ) identification. Which characteristics matter most.
- FMEA at design stage. What could go wrong.
- Process capability targets. Cpk per CTQ.
- Poka-yoke at workstations. Mistakes impossible or instantly visible.
- SPC monitoring. Process stays in control over time.
- Continuous improvement. Reduce variation further.
How QbI works in practice
- Specification of inspection points. Where to check.
- Sampling plans. How much to check.
- Detection methods. Visual, dimensional, functional.
- Defect handling. Rework, scrap, concession.
- Trend reporting. Defect rates over time.
QbI catches what slipped through; QbD prevents the slip.
Where QbI still has a role
- Final acceptance for customer.
- Regulatory inspection requirements.
- Statistical verification of QbD-driven processes.
- Backup when QbD fails.
Even mature QbD plants still inspect. The difference is the inspection finds fewer defects.
The transition path
- Map current inspection points and defect rates.
- Identify the top recurring defects.
- Trace each to design or process root cause.
- Design out via poka-yoke or process change.
- Verify the change reduces the defect rate.
- Reduce or eliminate the inspection point.
- Repeat.
This is a multi-year program. The path is incremental.
Common mistakes
1. Adding inspection in response to every escape. QbI grows; QbD never gets attention.
2. QbD without QbI safety net. Even mature designs have failures; verification remains essential.
3. No investment in QbD. Pay inspection cost forever.
4. No transition plan. QbD vs QbI presented as binary; pragmatic mix not pursued.
What changes with QbD
- Defect rate drops.
- Inspection cost falls.
- Scrap and rework decline.
- OEE Quality improves.
- Customer experience improves.
How OEE relates
OEE Quality is the share of good parts. QbD raises this. Plants with mature QbD see Quality climb above 99% on many lines; QbI plants struggle to break 97%.
Common mistakes
1. Treating QbD as engineering-only. Operations must own implementation.
2. Treating QbI as quality-only. Production and engineering need to learn from the data.
3. No metric on inspection-to-design ratio. Hard to see drift toward QbI.
How a modern OEE platform supports both
A modern OEE platform tracks defects with reason codes, surfaces patterns that inform design changes, and reports inspection load vs production load.
Fabrico's OEE module tracks defect reason codes, surfaces patterns for design improvement, and reports inspection vs production effort.
See how Fabrico captures this automatically — explore OEE for manufacturing or book a demo.
Related reading
Frequently asked questions
Is QbI obsolete?
No. Inspection remains essential as verification and safety net.
How fast does QbD pay back?
Variable. Often 18-36 months on high-volume products.
Can existing products move to QbD?
Partially. Process changes can implement QbD principles even on existing designs.
Should I eliminate inspection?
No. Reduce as QbD matures, never to zero.
Who should own QbD?
Engineering and operations together. Quality supports.
Zaloguj
Blog
