Sampling and AQL (ISO 2859-1)
Sampling decides how many units an inspection looks at and when the sample is good enough to accept the lot. Quality Control supports four schemes, from checking everything to full ISO 2859-1 AQL tables with dynamic modification. The scheme is chosen per item on its quality settings, and may be overridden by an inspection plan.
The four schemes
| Scheme | How many are inspected | Suits |
|---|---|---|
| 100% inspection | Every unit in the lot. | Low volumes, high-value items, anything safety-critical. |
| Fixed quantity | The same number every time, whatever the lot size. | A simple house rule such as "check five". |
| Percentage of the lot | A share of the quantity received. | Bulk goods where the risk scales with volume. |
| AQL (ISO 2859-1) | From the published tables, by lot size and inspection level. | Regular supply where you want a defensible, standard rule. |
How AQL works here
The app implements single sampling under normal inspection. Two published tables do the work:
- Table 1 turns the lot size and the inspection level into a sample size code letter.
- Table 2-A turns that letter and the AQL into the sample size, the acceptance number and the rejection number, following the table's arrows where a cell has none of its own.
The AQL values available are 0.010, 0.015, 0.025, 0.040, 0.065, 0.10, 0.15, 0.25, 0.40, 0.65, 1.0, 1.5, 2.5, 4.0, 6.5 and 10.
Inspection levels
| Level | Use |
|---|---|
| General I | Reduced discrimination — smaller samples, less certainty. |
| General II | The normal choice, and the usual default. |
| General III | More discrimination — larger samples. |
| Special S-1 to S-4 | Small samples for destructive or expensive tests. |
Critical defects
Whatever the AQL, a defect classed Critical is accepted at zero. One critical defect in the sample rejects the lot.
Skip-lot (dynamic modification)
Suppliers that keep delivering well should not be inspected as hard as suppliers that do not. Skip-lot adjusts the severity automatically, per supplier or job worker and item:
| Severity | What happens | How it is reached |
|---|---|---|
| Normal | The scheme runs as configured. | The starting state. |
| Skipped | Lots pass without inspection; one in every K is still checked. | After N lots accepted in a row. |
| Tightened (100%) | Everything is inspected. | A rejected lot sends the next M lots here. |
N, K and M are set on the item's quality settings. A skipped lot is recorded with the status Skipped — it is not missing, it is a decision the rules took, and the trail shows it.
Skip-lot counts by supplier and item together. A good record with one supplier does not relax inspection of the same item from another.
Where it is set
Set the scheme and its numbers on Item quality settings. An inspection plan attached to the item can carry its own sampling for the characteristics it covers. The scheme actually used appears as the Method on the inspection, so what a given check followed is always on the record.