The choices that set a sample size
| Choice | What it means | Effect |
|---|---|---|
| Lot size | How many pieces are in the delivery | Larger lots usually call for larger samples, but not in proportion |
| Inspection level | How strict the plan is | Tighter levels mean larger samples and more rejected lots |
| Defect classes | Critical, major, minor | Critical defects get stricter limits |
| Accept limit | How many defects in the sample you will tolerate | Zero is strict; more allows some defects |
Define defects first
A sampling plan is meaningless until the defects are defined.
- Critical: could cause injury or damage: missing carbide, cracked body, wrong hole pattern.
- Major: stops fitting or use: dimensions out of tolerance.
- Minor: cosmetic or marking issues.
Write the list with photographs, so that two inspectors reach the same decision.
A way to write the rule
Write the rule as numbers, not as a feeling about quality.
- Pick a sampling table or plan with your adviser and record its name and level.
- Write the accept and reject numbers by defect class.
- Say what happens on rejection: sort the lot, return it, or ask the supplier to re-inspect.
- Agree the rule with the supplier before the order.
Practical points
The practical points decide whether the sample represents the lot.
- Sample randomly from several cartons, not just the top of the pallet.
- Measure with calibrated tools and record serial numbers of tools.
- Keep failed samples.
Clear Roads’ standard-specification project lists accept/reject procedures for inserts as part of its scope, which shows that agencies treat inspection rules as part of the specification.
Keeping records
Record the lot number, date, sample size, tool used, defects found and decision for every delivery. After a season, count the rejected lots by supplier and defect type, and review the results with each supplier. Records turn inspection from a cost into a way to improve supply, and they give you evidence if you need to claim a refund or replacement.
Boundary: general explanation; no standard text was read and no sample sizes are given. Choose a published sampling standard with a quality adviser.
Sources and further reading. External references are dated; where a figure is quoted, the link shows where it came from.
Clear Roads: Standard Specifications for Plow Blades with Carbide Inserts (Project 17-02) (completed April 2020) ↗Can we combine several production lots into one incoming inspection sample? ↗Sampling and Pre-Production Approval for Custom Plow Edges ↗SENTHAI quality control ↗EN 10204 Type 2.2 vs 3.1: Which Certificate Do You Actually Need for Edge Steel? ↗Prop 65 and Carbide Products: Why a Warning Label May Appear on California Shipments ↗What a Standard Carbide-Blade Specification Covers: A Quote Checklist ↗
