Duplicate measures
Two precision statistics dominate duplicate QC, and mixing them up silently doubles or halves every threshold:
- HARD = |x₁−x₂| / (x₁+x₂) × 100
- RPD = |x₁−x₂| / mean × 100 — exactly 2× HARD
A "10% pulp criterion" means opposite things depending on which statistic is meant. Always label which one a chart or threshold uses.
The tolerance ladder
Precision degrades up the sampling chain, so tolerance widens with it:
| Duplicate type | Typical tolerance (RPD) | What it monitors |
|---|---|---|
| Field / twin | 30% | sampling + prep + analysis + geology |
| Coarse / prep | 20% | prep + analysis + geology |
| Pulp / lab | 10% | analysis only |
The better form: population rules
Per-pair pass/fail on nuggety mineralisation generates meaningless failures. The population form ranks pairs by HARD and asks what fraction beat the line: pulp 90% under 10%, coarse 80% under 10%, field 70% under 10% — the "90/80/70" ranked-HARD rule. Aggregate variants ("no more than 10% of pairs outside tolerance") serve the same purpose.
For precision as a function of grade, the Thompson–Howarth method (pairs sorted by concentration, grouped in 11s, median-based regression) is the standard chart — use robust regression, never ordinary least squares.
In geoDB
geoDB judges duplicate pairs on RPD, with warnings out to 1.5× the pass threshold:
| Duplicate type | geoDB pass threshold |
|---|---|
| Field / twin | 20% |
| Coarse / prep | 20% |
| Pulp / lab | 10% |
Our field threshold is tighter than the industry's 30% — deliberate, and a change we'd measure before making.
A project can set a per-element override (e.g. Au = 15%). An override tightens any tier, and may loosen only the field tier: field tolerance is your deposit's variability, while the coarse and pulp tolerances are the lab's precision, which a per-element preference shouldn't relax.
Pairs near the detection limit are judged on absolute difference instead — see Duplicates near DL. Each row on the certificate page says which test produced its verdict.
The duplicate charts plot RPD; ranked-HARD and Thompson–Howarth plots aren't built into the pages yet — ask Petra when you need one.