Capability Ratio (Balance vs Tolerance)

Capability ratio (balance vs tolerance) is the ratio between the tolerance band's half-width and the balance's readability d: R = tolerance ÷ d. The rule of thumb — d ≤ tolerance ÷ 10 (or equivalently R ≥ 10) — comes from measurement-systems-analysis theory (AIAG MSA, ISO 22514-7) and says the measurement instrument should have at least 10× finer resolution than the specification it is being used to judge. The principle: if the instrument's smallest discriminable unit is the same size as the tolerance, every measurement looks identical and the spec is not being measured, only labelled.

Applied to weighing: a balance with d = 0.1 mg used to enforce a ±1 mg tolerance has R = 10 — at the baseline. The same balance enforcing a ±2 mg tolerance has R = 20 — comfortable. The same balance enforcing a ±0.5 mg tolerance has R = 5 — caution band, may be acceptable under documented QRM with tighter PQ controls. Enforcing a ±0.2 mg tolerance has R = 2 — unworkable. The band exists in the MMR but the balance cannot tell the difference between 0.498, 0.500, and 0.502 mg.

The 10:1 rule is not arbitrary. It is the rounded-down version of the gauge-R&R guidance in AIAG MSA (and mirrored in ISO 22514-7) that the measurement system's total variability should consume less than 10% of the tolerance window. With d as the proxy for measurement resolution and ±tol as the spec window:

There are two ways to express the denominator of the capability ratio, and they answer slightly different questions. Both are valid and a defensible balance qualification documents both.

A formulator specifies an active charge of 250 mg with a tolerance of ±2% (±5 mg) per ICH Q8 design-space analysis. Available balances:

The dispense engine pre-flight queries balance master, computes R_d for each qualified balance against the charge's tolerance, filters for min-weight pass + max-load pass, ranks by best-fit (typically lowest R_d ≥ 10 to conserve premium balances for charges that need them), and pre-assigns the optimal balance. In the example above, B-204 is assigned; B-101 is reserved for sub-100 mg charges where it is actually needed; B-308 + B-412 are refused. If no qualified balance passes, the WO release fails — the operator cannot manually override into an unworkable assignment.

1. Balance assigned by tradition — 'we always use B-308 for this product' regardless of band; capability ratio never checked; weighments land 'in-band' because the balance cannot distinguish; latent OOS contributor; surfaces in §211.192 review when actual mass variability is investigated.
2. R_d passes but R_σ fails — balance specs claim d = 0.1 mg but PQ-measured σ_rep on the production floor is 0.5 mg; R_d says 10 but R_σ says 0.7; chronic mid-band variability that the band 'permits' but the formulation cannot tolerate.
3. Capability ratio computed against nominal target's tolerance, not the post-adjustment target — after PF + SBF + LOD + overage the actual target shrinks; tolerance (if absolute g) stays the same and ratio looks fine, but the relative tolerance has shrunk and the band may now fall below balance-resolution threshold.
4. No capability-ratio gate in the WO release engine — operators can pick any qualified balance from a dropdown; sub-10:1 assignments happen routinely; no audit trail of the assignment decision; no documented rationale.
5. Caution-band (R = 4-10) used without QRM documentation — assignment passes a soft warning; no formal ICH Q9 risk assessment + no compensating PQ tighten + no heightened SPC monitoring; first inspector question 'why R = 6 here?' has no answer.
6. Splitting a charge to evade capability ratio — large charge split into multiple sub-charges so each individually meets ratio, but cumulative chain has √n × individual uncertainty; in some cases an unsplit charge on a coarser balance with R = 8 produces less variability than a split charge on a finer balance with R = 12 each.
7. Same balance enforcing wildly different tolerances across products — a 4-place analytical balance used for ±5 mg (R = 50, premium) AND ±0.3 mg (R = 3, unworkable); capability ratio audit per balance per product surfaces the mismatch; usually the result of a single 'preferred balance' list with no per-product gate.
8. Capability ratio recomputed only at MMR approval, never at WO release — if a balance's PQ-measured σ_rep drifts upward (HVAC retrofit, neighbouring equipment, room reconfiguration) the R_σ collapses but the WO release engine still uses cached R_d; capability-ratio compliance gap goes undetected until OOS investigation surfaces it.

• Capability-ratio compliance % — fraction of WO-released charges where the assigned balance satisfies R_d ≥ 10 against the post-adjustment tolerance; target 100% (any sub-10 is either caution-band-with-QRM or a release-gate violation).
• Caution-band utilisation % — fraction of WO-released charges in the R = 4-10 caution band; tracked as a portfolio metric; sustained high rate indicates a balance-portfolio gap (need finer balances) or a tolerance-portfolio gap (need wider bands).
• Caution-band QRM compliance % — fraction of caution-band assignments with documented ICH Q9 QRM + compensating controls; target 100%; missing QRM is a §211.100 + ICH Q9 finding.
• R_σ-vs-R_d divergence per balance — for each balance, R_σ ÷ R_d trend; values approaching 1 are healthy; values << 1 indicate the environment has degraded σ_rep relative to d; investigation trigger.
• Sub-resolution band attempts (count) — count of WO releases that attempted to assign a balance with R_d < 4; target 0; the dispense engine refuses but any non-zero attempt indicates an MMR / balance-master gap.
• Capability-ratio recomputation lag — time between PQ σ_rep update and capability-ratio cache refresh in the WO-release engine; target ≤24 hours; sustained lag is a master-data-pipeline gap.
• Per-product capability index Cms — ISO 22514-7 measurement capability index Cms ≥ 1.33 for critical CQA-linked weighments; sustained Cms < 1.33 is a control-strategy revisit trigger.
• Premium-balance conservation index — fraction of charges assigned to a finer balance than necessary (R_d > 30); too-high values waste premium-balance availability for charges that actually need it; helps the engine balance fit vs conservation.

1. Balance master carries d + sigma_rep (per-PQ-measured) + qualified_min_weight + max_load + last_PQ_date + capability_attestation pointer.
2. MMR tolerance cells carry tolerance_unit + tolerance_low + tolerance_high; the engine computes the post-adjustment tolerance per charge per lot at WO-release time.
3. WO-release engine computes R_d AND R_σ per candidate balance against the post-adjustment tolerance; filters by min-weight pass + max-load pass + calibration-valid; ranks by best-fit (lowest R_d ≥ 10 to conserve premium balances) + R_σ ≥ 4 hard floor.
4. Sub-4 R_d is a hard block — RLS rejects WO release writes that assign a balance with R_d < 4 against the tolerance; service-role-key not exempt. Caution band R_d 4-10 requires a paired QRM document reference; the release gate refuses without one.
5. Kiosk widget shows R_d + R_σ + caution badge (if applicable) on the weighment widget — the operator sees the capability ratio at the moment of weighment, not buried in a master-data screen.
6. PQ σ_rep updates write through to balance master + invalidate capability-ratio cache + recompute open WOs' assignments; any open WO whose capability ratio collapses below threshold opens a deviation candidate + pages the production supervisor.
7. Cms (ISO 22514-7) computed quarterly per balance per product class; trended on the Quality dashboard; values < 1.33 trigger control-strategy revisit ticket.
8. Audit trail captures the capability-ratio decision at WO release: assigned_balance_id, R_d, R_σ, qrm_doc_id (if caution band), alternative_balances_considered, ranking_rationale.
9. Per-product per-balance per-operator capability-ratio compliance + caution-band utilisation + sub-resolution attempts surface in the quarterly ICH Q10 §3.2.5 review.
10. Master-data governance: capability-ratio policy (10:1 baseline + 4 hard floor + caution band threshold + Cms ≥ 1.33 target) is change-controlled in a single policy table; edits require Quality VP + Manufacturing VP e-sigs.