Skip to content

Practical guide · Quality management

Quality assurance vs quality control: the difference, with a plant example

Short answer

Quality assurance (QA) is the planned work that makes a process able to deliver good product: procedures, training, audits, controls. Quality control (QC) is checking the product itself, by inspection and test, to find what is out of specification. QA prevents defects. QC finds them.

Quality assurance and quality control are two parts of quality management. Quality assurance is the system side: it plans and checks the process so that product comes out right the first time, through procedures, training, supplier control, audits and corrective action. Quality control is the product side: it measures parts, lots or batches against the requirement and decides what passes. In short, QA works on the process before and between runs, and QC works on the output during and after it. A plant needs both, because control alone finds the same defect again and again, and assurance alone never proves a given lot is good.

Last updated:

QA vs QC in one table

The ISO 9000 vocabulary, which underlies the ISO 9001 quality standard, describes quality assurance as the part of quality management focused on providing confidence that quality requirements will be fulfilled, and quality control as the part focused on fulfilling them. Check the current edition for the exact wording. Everyday practice reads like this.

Quality assurance and quality control compared
Quality assurance (QA)Quality control (QC)
Question it answersCan this process make good product, and can we show it?Is this part, lot or batch within the requirement?
FocusThe process and the systemThe product
TimingBefore and between runs; ongoingDuring and after production, at receiving and before shipping
NaturePreventiveDetective
Typical activitiesWritten procedures, training, internal audits, supplier approval, error-proofing, corrective action, document controlIncoming, in-process and final inspection, measurement, testing, sampling, control charts, hold and release decisions
Typical outputProcedures, audit findings, a trained workforce, a corrective action closedPass or fail, measured values, a nonconformance report, a defect count
Who does itQuality engineers, quality managers, auditors, process ownersInspectors, operators doing self-check, lab technicians

A common split. Plants organize the two differently, and one person often does both.

The two are not rivals. QC tells you whether the product is good today and gives QA the data to see where the process is weak. QA then changes the process so QC has less to catch. Related ideas sit on the same line: quality at source pushes QC to the operator, poka-yoke turns a QA decision into a physical guard, and statistical process control uses QC data to steer the process.

Worked example: a lot of machined shafts

The numbers are made up for this page and are illustrative, not from a real plant.

The QC side

A cell makes 1,000 steel shafts a day. At final inspection the inspector measures 50 shafts from the lot against the diameter limit and finds 3 over the upper limit. 3 out of 50 is 6%. If the sample represents the lot, about 6% of 1,000, which is 60 shafts, are out of tolerance. QC does its job: it flags the lot, the 60 suspect shafts are held and the lot is sorted. This is containment, and it records the problem in a non-conformance report.

The QA side

Sorting does not stop tomorrow's lot from having the same problem. The quality engineer reviews the process. The oversize shafts all come from the end of the shift, and the tool-wear check is not on any schedule, so the insert runs until it is noticed. The corrective action is a rule: change the insert every 400 parts and add the change to the written procedure, with a one-point lesson for operators. At 1,000 parts a day that is a change at part 400 and part 800, two changes a day. If each takes 6 minutes, that costs 2 times 6, which is 12 minutes of run time a day, in return for taking the oversize shafts out of the process. The engineer then watches the next weeks of inspection data. If the oversize rate falls, the QA change worked. If it does not, the cause was something else.

That sequence is the whole relationship: QC detects, the cause is found with root cause analysis, QA changes the system, and QC confirms the result.

How QA and QC work together in a plant

  1. 1

    QA sets the requirement and the method

    Specifications, control plans, inspection methods and sampling plans are written and approved. Gauges are calibrated and the people who use them are trained.

  2. 2

    QC checks the work

    Incoming inspection on material, in-process checks on the line, final inspection before shipping. An out-of-limit result stops the lot and raises an NCR.

  3. 3

    QA investigates the pattern

    A single defect is a QC event. The same defect three times is a QA problem. Pareto the defects, find the root cause and open a corrective action. The problem-solving methods guide shows which method fits.

  4. 4

    QA verifies the fix is real

    An internal audit or a layered process audit checks that the new procedure is followed, and the QC data shows the defect rate moving.

  5. 5

    Both feed management review

    Defect rates, audit results and corrective action status go to the leadership team on a fixed cadence.

Common mistakes

  • Treating inspection as the quality system. If the answer to every defect is more inspection, the plant is buying QC to cover a QA gap, and inspection alone misses defects.
  • Using the two words as synonyms in a procedure. Auditors and customers read them as different jobs. Name the activity: audit, inspection, test.
  • Keeping QA in an office. A procedure nobody follows is not assurance. Check on the floor that the work matches the document.
  • Skipping the loop back. QC data that never reaches the corrective action process makes the same lot a new surprise every week.
  • Ignoring the gauge. QC is only as good as the measurement, so a gauge that has not been checked is a QA failure. See measurement system analysis.

How LeanSuite fits

LeanSuite is shop-floor software and works alongside your quality management system. On the QC side, its quality improvement module logs quality issues from the floor, and non-conformance management moves a defect report into a workflow. On the QA side, the corrective action log keeps one list of problems with an owner and a due date, and audit management runs scheduled audits. It does not replace an inspection instrument or a laboratory system. See the quality management hub for more.

Go deeper

This page is the short version. These guides cover each part in full.

All our articles on this subject are in the quality management topic.

Free templates and tools

Key terms

Lean glossary

Doing this in LeanSuite

See it on your own lines

Bring the problem you are working on to a 45-minute demo. Or look around with sample data first, or pilot LeanSuite on your own lines for 90 days, with the setup done by our team.

Book a 45-minute demoClick through the interactive demoStart a 90-day pilot

FAQ

Questions about quality assurance vs quality control

  1. Quality assurance works on the process so that good product is made in the first place: procedures, training, audits and corrective action. Quality control checks the product itself by inspection and test to find what is out of specification. QA is preventive and QC is detective.

  2. Inspection is quality control, because it checks the product. The audit of whether inspections are done to the written method, and whether the gauges are calibrated, is quality assurance.

  3. Yes, and in small plants one person often does. The two jobs are still different. Some quality systems expect the person who checks the work not to be the same person who audits the system, so check the requirements that apply to you.

  4. QA comes first in time because it sets up the process, the method and the training. QC then checks the output. In practice they run together: QC findings feed QA changes, which QC then verifies.

  5. The logic is the same. QA covers the practices used across the development process, such as reviews, standards and test planning. QC is the testing of the build itself to find defects before release. Terms vary between teams, so define them in your own plan.

Related guides and tools

More method guides

Or browse the topics, the full library and the lean glossary.

Rather see it on your own problem?

Bring the problem you are stuck on, and we will show you on a call how LeanSuite handles it.

Schedule Demo

Pass it on

We want LeanSuite to be the best place on the internet for lean help. If this was useful, send it to someone on your team or in your network who needs it.