QA vs QC: quality assurance vs quality control
Quality assurance (QA) is everything you do to make a conforming result likely: the procedures, the plans, the competency, the acceptance criteria agreed before work starts. Quality control (QC) is everything you do to confirm the result actually conforms: the inspections, measurements, tests and witness points applied to the work itself.
The shortest honest answer: QA is process-oriented and preventive, QC is product-oriented and detective. QA asks "is the way we are working capable of producing a conforming outcome?" QC asks "does this specific weld, batch, document or installation meet the requirement?"
On a real capital project the two are inseparable. QA writes the inspection and test plan; QC executes the inspections it specifies. QA defines what an acceptable weld is; QC measures the weld. When people say "QA/QC" they usually mean the combined function that does both.
- 01QA is preventive and process-focused. QC is detective and product-focused.
- 02QA activities happen mostly before and around the work; QC activities happen on the work itself.
- 03An ITP is a QA document; the inspection records it generates are QC evidence.
- 04Both are required by ISO 9001 — the standard does not use the terms, but clauses 8.1, 8.5 and 8.6 cover planning (QA) and release of outputs (QC).
- 05Independent QA/QC matters most where the party doing the work also grades its own homework.
What quality assurance actually means
Quality assurance is the planned, systematic set of activities that gives confidence a requirement will be met. It is a bet placed before the work happens: if the specification is clear, the procedure is qualified, the welder is certified, the supplier was assessed and the acceptance criteria are agreed in writing, then a conforming result is far more likely than not.
Nothing in that list involves touching the product. QA is documentation, planning, competency, qualification, audit and system design. Its output is confidence, not a measurement.
- Writing the project quality plan and the inspection and test plan (ITP)
- Qualifying welding procedures and verifying welder certification
- Pre-qualifying and auditing suppliers before a purchase order is placed
- Defining acceptance criteria, hold points and witness points with the client
- Internal audits of whether the system is being followed
- Training, competency records and document control
What quality control actually means
Quality control is the part where someone checks the real thing against the stated requirement and records what they found. It produces objective evidence: a dimension, a hardness value, a radiograph, a pressure test chart, a signed release note. If a QC activity does not generate a record, it did not happen as far as an auditor or a client is concerned.
- Dimensional and visual inspection of fabricated items
- Non-destructive testing — RT, UT, MT, PT — and review of the reports
- Material verification, PMI and mill certificate review against the specification
- Hydrostatic and pressure testing, functional testing, FAT and SAT
- Witnessing hold points at a vendor's shop before release
- Raising a non-conformance report when something fails the criterion
QA vs QC side by side
| Quality assurance (QA) | Quality control (QC) | |
|---|---|---|
| Purpose | Prevent non-conformance | Detect non-conformance |
| Focus | The process and the system | The product, batch or deliverable |
| Timing | Before and alongside the work | During and after the work |
| Typical output | Plans, procedures, ITPs, audit reports, qualifications | Inspection reports, test certificates, NCRs, release notes |
| Who does it | Quality manager, QA engineer, lead auditor | Inspector, NDT technician, QC engineer |
| Question answered | Are we set up to get this right? | Did we get this one right? |
| Cost profile | Cost of prevention — paid up front | Cost of appraisal — paid per item inspected |
| Failure mode | A capable process nobody follows | Inspection that finds problems too late to fix cheaply |
One example, traced end to end
Take a single pressure-retaining weld on a fabricated spool for a plant project. Here is where each activity falls.
- Step 1QA
Requirement agreed
The specification, code and acceptance criteria for the weld are fixed in the purchase order and reflected in the ITP.
- Step 2QA
Capability qualified
The welding procedure is qualified, the welder's certification is verified as current, and the NDT contractor's technicians are approved.
- Step 3QA
Hold point set
The ITP places a client hold point after NDT and before painting, so the weld cannot be hidden before it is accepted.
- Step 4QC
Weld inspected
Visual inspection is carried out, radiography is performed and the film is interpreted against the code.
- Step 5QC
Result recorded
The inspection report and NDT report are issued, referenced to the weld number and filed against the ITP line.
- Step 6QC
Failure handled
A rejectable indication triggers a non-conformance report: the item is contained, dispositioned and re-inspected after repair.
- Step 7QA
Cause addressed
Three similar rejects in a month trigger a corrective action on the procedure, the technique or the operator — back in QA territory.
Four things people get wrong
"QA is just paperwork"
The paperwork is the mechanism. A qualified procedure and an agreed acceptance criterion are what make an inspection meaningful — without them, an inspector is comparing the work to an opinion.
"QC is optional if QA is strong"
No process is perfect. QC is how you learn that the process drifted, and it is the evidence a client or certification body will ask to see.
"QA/QC is one job title"
On smaller projects one person often does both, which is workable — until that person has to audit their own inspection records. Independence is what breaks first.
"Testing equals quality control"
Testing is one QC method. Document review, material traceability checks and witness points are QC too, and they catch different failures.
Where the standard puts each one
ISO 9001 never uses the phrases "quality assurance" and "quality control" as a pair of defined terms, which is one reason the distinction gets muddled. The activities are still there, split across the operational clauses.
| Clause | What it covers | QA or QC |
|---|---|---|
| 8.1 Operational planning and control | Determining requirements, criteria and the resources needed before work starts | QA |
| 8.4 Externally provided processes | Supplier evaluation, selection and control | QA |
| 8.5.1 Control of production and service provision | Working under controlled conditions with monitoring at defined stages | Both |
| 8.6 Release of products and services | Verifying requirements are met before release, with evidence of conformity | QC |
| 8.7 Control of nonconforming outputs | Identifying, containing and dispositioning what fails | QC |
| 9.2 Internal audit / 10.2 Corrective action | Checking the system works and fixing causes | QA |
How much of each does a project need?
The practical answer is set by consequence and by how late a defect would be discovered. A buried pipe weld, a vessel internal or a structural connection that will be enclosed deserves QA effort up front and a hard hold point, because the cost of finding the problem afterwards is measured in weeks of programme. A low-consequence commodity item can be released on documentation review alone.
Questions we get on this topic
What is the main difference between QA and QC?
QA is preventive and works on the process — plans, procedures, qualifications and audits designed to make a conforming result likely. QC is detective and works on the product — inspections, measurements and tests that confirm a specific item meets the requirement.
Is an inspection and test plan QA or QC?
The ITP itself is a QA document: it plans what will be verified, at which stage, against which criterion, and who attends. The inspection and test records generated when the ITP is executed are QC evidence.
Does QA come before QC?
Generally yes. QA sets the requirement, the method and the acceptance criteria; QC then verifies the work against them. In practice the two run in parallel through a project, with QC findings feeding corrective action back into QA.
Can the same person do QA and QC?
On small scopes, often yes. The constraint is independence: whoever verifies conformity should not be the person who produced the work or who owns the schedule pressure to release it.
What does QA/QC mean as a single term?
QA/QC is the combined quality function on a project — the team responsible both for the quality planning documents and for the inspection and test activity that verifies the work.
Need independent QA and QC on a project?
We build the quality plans and ITPs, then execute the inspection and surveillance that verifies the work — with evidence your client and certification body will accept.