Unit Price / Best-Value Comparator mechanism

Model scope

The arithmetic formula is standard, but the specification is not complete enough for unambiguous implementation. Exact ambiguities requiring review are the conversion-factor source, target-unit selection, currency handling, invalid-unit behavior, precision and tie-breaking rules, and the meaning of "best value." An honest label should state that the instrument computes lowest price per comparable unit only; it does not assess quality, quantity suitability, nutrition, or total consumer value.

• The division formula is correct only when every quantity is expressed in compatible units and every price uses the same currency.
• The phrase "normalized to same unit" does not specify conversion factors, a target unit, or handling for incompatible units.
• "Best value" is ambiguous: the formula ranks lowest monetary cost per quantity, not overall product value, quality, or utility.
• No rounding or tie-breaking rule is specified for displayed unit prices and rankings.

Units and conventions

• All prices in a comparison use the same currency; test vectors use USD.
• Quantities are positive and represent comparable amounts of the same product dimension.
• Unit conversion is exact or uses an explicitly defined conversion factor; the target unit is selected before comparison.
• A lower price per normalized unit is ranked as better value.
• Prices and quantities are treated as numeric values before display rounding.

Inspectable vector

One of 4 source-bound review vectors:

{
  "inputs": {
    "options": [
      {
        "id": "A",
        "price": 5.98,
        "quantity": 2,
        "unit": "L"
      },
      {
        "id": "B",
        "price": 3.29,
        "quantity": 1,
        "unit": "L"
      }
    ],
    "target_unit": "L",
    "currency": "USD"
  },
  "expected_outputs": {
    "unit_prices": [
      {
        "id": "A",
        "price_per_unit": 2.99,
        "unit": "USD/L"
      },
      {
        "id": "B",
        "price_per_unit": 3.29,
        "unit": "USD/L"
      }
    ],
    "ranked_best_value": [
      "A",
      "B"
    ]
  },
  "working": "A: 5.98 USD / 2 L = 2.99 USD/L. B: 3.29 USD / 1 L = 3.29 USD/L. Since 2.99 < 3.29, ranking is A first, then B."
}

Failure boundary

BREAK_CLAIMED · SELF_FLAGGED

A zero quantity produces an undefined division, and mixed units require an explicitly defined normalization branch. Without validation and unit conversion, the comparator can produce undefined results or an incorrect ranking.

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche5/tranche5-results.json
• BOUNDARY_HAZARD_CATALOG.json