What quality control is
Quality control (QC) is the set of activities that examine a product, component or deliverable to find out whether it meets its requirements. It answers a direct question: is this particular item acceptable? Typical activities are inspection, measurement and testing, followed by a decision to accept, rework or reject.
QC is reactive in a useful sense. It detects defects that exist, so they can be contained before they reach a customer. It does not by itself make defects less likely to occur. That is the job of quality assurance, which looks at the process that produces the work.
QC versus QA
The two terms are often confused, so a short comparison helps.
| Quality assurance | Quality control | |
|---|---|---|
| Focus | The process | The product or deliverable |
| Question | Are we working in a way likely to give good results? | Does this item meet its requirements? |
| Typical activities | Planning, standards, reviews, audits | Inspection, measurement, testing |
| Timing | Throughout, mostly preventive | At defined points, mostly detective |
Neither replaces the other. A strong process with no checks leaves you trusting that nothing went wrong. Heavy checking on a poor process means you keep finding the same problems and paying to sort them out. Many organisations use both, and in practice the boundary between them is not always sharp.
Start with acceptance criteria
You cannot inspect against a vague goal. Acceptance criteria state the conditions an item must meet to be accepted: dimensions and tolerances, functional behaviour, finish, performance limits, documentation to be supplied. Good criteria are specific enough that two different people would reach the same decision.
Criteria come from requirements, specifications, drawings, contracts and applicable standards. If a requirement cannot be checked, it should be clarified before work begins. The ideas behind acceptance criteria and writing requirements apply to hardware and software alike.
Inspection
Inspection means comparing the item against the criteria, by looking, measuring or using gauges. Common forms include:
- Visual inspection for surface defects, workmanship, labelling and assembly errors.
- Dimensional inspection using tools such as calipers, micrometers, gauges or coordinate measuring machines.
- Documentation checks to confirm that records, certificates and traceability data are present and consistent.
Inspection can take place on incoming materials from suppliers, during production, and at final release. Each point catches different problems. Catching a fault early in production generally avoids adding further work to an item that will be rejected anyway.
Measurement and its limits
Every measurement has some uncertainty, so a reading close to a limit needs care. If a part is specified at 10.00 mm with an allowed range of plus or minus 0.05 mm, then 9.95 to 10.05 mm is acceptable. A reading of 10.04 mm from an instrument that is itself uncertain by a few hundredths of a millimetre does not give a clear pass. Understanding tolerances and measurement uncertainty helps you decide how good the instrument needs to be relative to the tolerance being checked.
Instruments should be suitable for the task and kept in calibration according to the procedures that apply to your organisation. A precise instrument that has drifted can be worse than a rough one that is known to be accurate.
Testing
Testing checks behaviour rather than appearance or size. Examples are load tests on mechanical parts, electrical tests on boards, burn-in or environmental tests, and software testing at the unit and system levels. Prototype testing is often done methodically in development, while production testing checks each unit or a sample for consistency.
A test is only as good as its method. Define the setup, the expected result and the pass or fail limit before running it, and record the outcome with the conditions. Where a test cannot show that something is safe, do not treat a pass as proof of safety.
Sampling
Checking every item is not always practical or possible. Destructive tests, for example, consume the item being tested. In those cases sampling is used: a portion of a batch is checked, and the result is used to judge the whole. Sampling plans are defined in published standards and in statistical methods. They involve a risk of accepting a bad batch or rejecting a good one, which is why the plan should be chosen with someone who understands the statistics and the risks of your industry, rather than made up on the day.
Handling nonconforming items
When an item fails, the response should be planned in advance:
- Identify and separate it, so it cannot be mistaken for a good item.
- Decide on the disposition: rework, repair, use as is where permitted, or scrap. In many settings this decision belongs to a defined authority and must be recorded.
- Investigate the cause, using methods such as root cause analysis, so the process can be corrected.
Skipping the third step turns QC into endless sorting. The findings should feed back into process changes and continuous improvement.
Recording results
Inspection and test records show what was checked, against what, when, by whom and with what result. They support traceability and can matter in disputes, recalls or investigations. Record actual measurements rather than only “pass”, where practical, because trends in the numbers may warn you of drift before anything fails.
Common pitfalls
- Inspecting against criteria that nobody wrote down.
- Relying on inspection alone to produce quality; inspection finds defects but does not create good work.
- Pressure to pass borderline items when a deadline is near. Honest reporting matters, and this is an area where engineering ethics applies directly.
- Using uncalibrated or unsuitable instruments.
- Not feeding results back into the process.
QC requirements differ by country, industry and customer, and some fields have mandatory inspection regimes. Follow the standards and professionals that apply to you. This article is general educational information, not engineering or compliance advice.