Acceptance sampling is a way of deciding whether a whole batch is good enough by inspecting only part of it. The batch is called the lot: a defined quantity of product made under similar conditions, such as one purchase order of finished shirts delivered to the final-inspection room. The sample is the smaller number of units you actually pull out and examine.
The sampling plan tells you three things: how many units to inspect, the accept number (Ac), and the reject number (Re). If the count of defective units in the sample is at or below Ac, the lot is accepted. If the count reaches Re, the lot is rejected. Re is always Ac plus one, so there is never a gap between the two.
The sample has to represent the lot. Pull shirts from cartons spread across the whole delivery, from the top, middle and bottom of each carton, and across the sizes and colours in the order. A sample taken from the two nearest cartons tells you about those cartons, not about the lot.
Standard Procedure
- 1Confirm the lot boundaries: one order, one style, one production run.
- 2Read the plan: sample size, Ac and Re for each defect class.
- 3Draw the sample randomly across cartons, sizes and colours.
- 4Inspect each sampled unit and count defective units.
- 5Compare the count to Ac and Re and record the decision.
Lesson Summary
- ✓A lot is the batch being judged; the sample is the part you inspect.
- ✓Accept if defectives are at or below Ac; reject if they reach Re.