diff --git a/domain/epc_prediction/validation.py b/domain/epc_prediction/validation.py
new file mode 100644
index 00000000..d778246e
--- /dev/null
+++ b/domain/epc_prediction/validation.py
@@ -0,0 +1,159 @@
+"""Component Accuracy aggregation for EPC Prediction (ADR-0030).
+
+The leave-one-out scorer, calculator-FREE on purpose: it holds out each SAP 10.2
+target, predicts it from its (all-vintage) Comparable Properties, and aggregates
+the per-component classification hits + geometry residuals from
+`compare_prediction`. This is the *primary*, calculator-independent signal — the
+end-to-end SAP / carbon / PE check (which needs the calculator) is layered on top
+by the runner. The same function backs both the committed ratcheting gate and the
+offline national battle-test (one scorer, two harnesses).
+
+Pure given the loaded cohorts: corpus IO (reading + mapping cert payloads) is the
+caller's job, so this is directly unit-testable.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from datetime import date
+from typing import Iterable, Iterator, Optional, Sequence
+
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from domain.epc_prediction.comparable_properties import (
+    Comparable,
+    PredictionTarget,
+    select_comparables,
+)
+from domain.epc_prediction.epc_prediction import EpcPrediction
+from domain.epc_prediction.prediction_comparison import compare_prediction
+
+# Only SAP 10.2 certs are valid held-out targets (ADR-0030) — the only vintage
+# with full-fidelity lodged components. The source cohort keeps all vintages.
+_SAP_10_2: float = 10.2
+
+
+def _empty_classification() -> dict[str, list[int]]:
+    return {}
+
+
+def _empty_residuals() -> dict[str, list[float]]:
+    return {}
+
+
+@dataclass
+class ComponentAccuracy:
+    """Aggregated leave-one-out Component Accuracy over a corpus.
+
+    `classification` maps a component name to [hits, applicable-total] (a
+    not-applicable `None` hit is excluded from the total); `residuals` maps a
+    numeric component to its signed (predicted − actual) values. `targets` counts
+    the held-out SAP 10.2 properties scored.
+    """
+
+    classification: dict[str, list[int]] = field(
+        default_factory=_empty_classification
+    )
+    residuals: dict[str, list[float]] = field(default_factory=_empty_residuals)
+    targets: int = 0
+
+    def rate(self, component: str) -> Optional[float]:
+        """The classification hit-rate for a component, or None when nothing was
+        applicable."""
+        hits, total = self.classification.get(component, [0, 0])
+        return hits / total if total else None
+
+    def mean_abs_residual(self, component: str) -> Optional[float]:
+        """Mean absolute residual for a numeric component, or None when empty."""
+        values = self.residuals.get(component, [])
+        return sum(abs(v) for v in values) / len(values) if values else None
+
+
+def _recency_key(comparable: Comparable) -> tuple[date, str]:
+    return (
+        comparable.registration_date or date.min,
+        comparable.certificate_number,
+    )
+
+
+def _latest_per_address(cohort: Sequence[Comparable]) -> list[Comparable]:
+    """One held-out property per address — the latest cert, the best ground
+    truth. Comparables with no address each stand alone."""
+    latest: dict[str, Comparable] = {}
+    standalone: list[Comparable] = []
+    for c in cohort:
+        if c.address is None:
+            standalone.append(c)
+        elif c.address not in latest or _recency_key(c) > _recency_key(
+            latest[c.address]
+        ):
+            latest[c.address] = c
+    return list(latest.values()) + standalone
+
+
+def iter_predictions(
+    cohorts: Iterable[Sequence[Comparable]],
+    *,
+    target_sap_version: float = _SAP_10_2,
+) -> Iterator[tuple[EpcPropertyData, EpcPropertyData]]:
+    """Yield `(predicted, actual)` for every SAP-`target_sap_version` held-out
+    target across the cohorts — the single leave-one-out orchestration the
+    Component Accuracy scorer and the runner's calculator end-to-end both consume
+    (ADR-0030: one scorer, two harnesses). A target is held out by whole address
+    (so a re-lodgement can't leak) and predicted from its all-vintage cohort."""
+    predictor = EpcPrediction()
+    for cohort in cohorts:
+        for held_out in _latest_per_address(cohort):
+            if held_out.epc.sap_version != target_sap_version:
+                continue
+            others = [
+                c
+                for c in cohort
+                if c.address is None or c.address != held_out.address
+            ]
+            actual = held_out.epc
+            target = PredictionTarget(
+                postcode=actual.postcode,
+                property_type=actual.property_type or "",
+                built_form=actual.built_form,
+            )
+            comparables = select_comparables(target, others)
+            if not comparables.members:
+                continue
+            yield predictor.predict(target, comparables), actual
+
+
+def evaluate_component_accuracy(
+    cohorts: Iterable[Sequence[Comparable]],
+    *,
+    target_sap_version: float = _SAP_10_2,
+) -> ComponentAccuracy:
+    """Score Component Accuracy by leave-one-out over each postcode cohort —
+    aggregating the `compare_prediction` hits + residuals across every held-out
+    SAP-`target_sap_version` target. Calculator-free (the primary signal)."""
+    accuracy = ComponentAccuracy()
+    for predicted, actual in iter_predictions(
+        cohorts, target_sap_version=target_sap_version
+    ):
+        comparison = compare_prediction(predicted, actual)
+        accuracy.targets += 1
+        for name, hit in comparison.categorical_hits.items():
+            counter = accuracy.classification.setdefault(name, [0, 0])
+            if hit is not None:
+                counter[1] += 1
+                counter[0] += int(hit)
+        accuracy.residuals.setdefault("floor_area", []).append(
+            comparison.floor_area_residual
+        )
+        accuracy.residuals.setdefault("window_count", []).append(
+            float(comparison.window_count_residual)
+        )
+        accuracy.residuals.setdefault("total_window_area", []).append(
+            comparison.total_window_area_residual
+        )
+        accuracy.residuals.setdefault("building_parts", []).append(
+            float(comparison.building_parts_residual)
+        )
+        accuracy.residuals.setdefault("door_count", []).append(
+            float(comparison.door_count_residual)
+        )
+    return accuracy
diff --git a/harness/epc_prediction_corpus.py b/harness/epc_prediction_corpus.py
new file mode 100644
index 00000000..e52311e7
--- /dev/null
+++ b/harness/epc_prediction_corpus.py
@@ -0,0 +1,71 @@
+"""Load a postcode-clustered EPC corpus into Comparable cohorts (ADR-0030).
+
+The IO half of the EPC Prediction validation: read each postcode's cached cert
+payloads, map them through `EpcPropertyDataMapper.from_api_response`, and build
+`Comparable`s carrying the register metadata (address + registration date) the
+leave-one-out scorer needs to dedupe re-lodgements and hold out a whole address.
+A cert the mapper rejects (unsupported schema, malformed) is skipped, never fatal.
+
+Shared by the committed-fixture gate, the local validation script, and the
+offline national battle-test — the corpus directory differs, the loading does
+not. Layout: `<dir>/<POSTCODE>/<cert>.json` + `<dir>/_index.json`.
+"""
+
+from __future__ import annotations
+
+import json
+from datetime import date
+from pathlib import Path
+from typing import Any, Optional
+
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+from domain.epc_prediction.comparable_properties import Comparable
+
+
+def load_corpus(corpus_dir: Path) -> list[list[Comparable]]:
+    """Load every postcode cohort under `corpus_dir`. Returns one list of
+    Comparables per postcode (the unit the leave-one-out scorer iterates)."""
+    index_path = corpus_dir / "_index.json"
+    if not index_path.exists():
+        raise FileNotFoundError(
+            f"no corpus index at {index_path} — run a corpus fetch first"
+        )
+    index: dict[str, list[str]] = json.loads(index_path.read_text())
+    return [
+        _load_cohort(corpus_dir, postcode, certs)
+        for postcode, certs in index.items()
+    ]
+
+
+def _load_cohort(
+    corpus_dir: Path, postcode: str, certs: list[str]
+) -> list[Comparable]:
+    cohort: list[Comparable] = []
+    for cert in certs:
+        path = corpus_dir / postcode / f"{cert}.json"
+        if not path.exists():
+            continue
+        raw: dict[str, Any] = json.loads(path.read_text())
+        try:
+            epc = EpcPropertyDataMapper.from_api_response(raw)
+        except Exception:  # noqa: BLE001 — a bad cert must not abort the sweep
+            continue
+        cohort.append(
+            Comparable(
+                epc=epc,
+                certificate_number=cert,
+                address=_address(raw),
+                registration_date=_registration_date(raw),
+            )
+        )
+    return cohort
+
+
+def _address(raw: dict[str, Any]) -> Optional[str]:
+    value = raw.get("address_line_1")
+    return str(value).strip().upper() if value else None
+
+
+def _registration_date(raw: dict[str, Any]) -> Optional[date]:
+    value = raw.get("registration_date")
+    return date.fromisoformat(str(value)) if value else None
diff --git a/scripts/validate_epc_prediction.py b/scripts/validate_epc_prediction.py
index 18ee4bbb..797389d2 100644
--- a/scripts/validate_epc_prediction.py
+++ b/scripts/validate_epc_prediction.py
@@ -23,90 +23,24 @@ Corpus dir: $EPC_PREDICTION_CORPUS (default /tmp/epc_prediction_corpus).
 
 from __future__ import annotations
 
-import json
 import os
 import statistics
-from datetime import date
 from pathlib import Path
 from typing import Optional
 
 from datatypes.epc.domain.epc_property_data import EpcPropertyData
-from datatypes.epc.domain.mapper import EpcPropertyDataMapper
-from domain.epc_prediction.comparable_properties import (
-    Comparable,
-    PredictionTarget,
-    select_comparables,
+from domain.epc_prediction.validation import (
+    evaluate_component_accuracy,
+    iter_predictions,
 )
-from domain.epc_prediction.epc_prediction import EpcPrediction
-from domain.epc_prediction.prediction_comparison import compare_prediction
 from domain.sap10_calculator.calculator import Sap10Calculator, SapResult
+from harness.epc_prediction_corpus import load_corpus
 
-# Target-cert spec gate: only SAP 10.2 certs (schema 21.0.x) carry full-fidelity
-# lodged components + a same-spec lodged figure to check against (ADR-0030). The
-# source cohort keeps all vintages — components are methodology-agnostic.
-_SAP_10_2: float = 10.2
 _KG_PER_TONNE: float = 1000.0
 
 CORPUS = Path(os.environ.get("EPC_PREDICTION_CORPUS", "/tmp/epc_prediction_corpus"))
 
 
-def _load_cohort(postcode: str, certs: list[str]) -> list[Comparable]:
-    """Map a postcode's cached cert payloads to Comparables, skipping any the
-    mapper rejects (unsupported schema, malformed). Address + registration date
-    come straight off the cached payload (the register metadata) so the harness
-    can dedupe re-lodgements and hold out a whole address."""
-    cohort: list[Comparable] = []
-    for cert in certs:
-        path = CORPUS / postcode / f"{cert}.json"
-        if not path.exists():
-            continue
-        raw = json.loads(path.read_text())
-        try:
-            epc = EpcPropertyDataMapper.from_api_response(raw)
-        except Exception:  # noqa: BLE001 — a bad cert must not abort the sweep
-            continue
-        cohort.append(
-            Comparable(
-                epc=epc,
-                certificate_number=cert,
-                address=_address(raw),
-                registration_date=_registration_date(raw),
-            )
-        )
-    return cohort
-
-
-def _address(raw: dict[str, object]) -> Optional[str]:
-    value = raw.get("address_line_1")
-    return str(value).strip().upper() if value else None
-
-
-def _registration_date(raw: dict[str, object]) -> Optional[date]:
-    value = raw.get("registration_date")
-    return date.fromisoformat(str(value)) if value else None
-
-
-def _ground_truth_properties(cohort: list[Comparable]) -> list[Comparable]:
-    """Collapse a postcode's certs to one held-out property per address — the
-    latest cert, the best ground truth. Comparables with no address each stand
-    alone."""
-    latest: dict[str, Comparable] = {}
-    standalone: list[Comparable] = []
-    for c in cohort:
-        if c.address is None:
-            standalone.append(c)
-        elif c.address not in latest or _recency(c) > _recency(latest[c.address]):
-            latest[c.address] = c
-    return list(latest.values()) + standalone
-
-
-def _recency(comparable: Comparable) -> tuple[date, str]:
-    return (
-        comparable.registration_date or date.min,
-        comparable.certificate_number,
-    )
-
-
 def _result(
     calculator: Sap10Calculator, epc: EpcPropertyData
 ) -> Optional[SapResult]:
@@ -123,111 +57,55 @@ def _co2_tonnes(result: SapResult) -> float:
 
 
 def main() -> None:
-    index_path = CORPUS / "_index.json"
-    if not index_path.exists():
-        raise SystemExit(f"no corpus at {CORPUS} — run fetch_epc_prediction_corpus.py")
-    index: dict[str, list[str]] = json.loads(index_path.read_text())
-
+    cohorts = load_corpus(CORPUS)
     calculator = Sap10Calculator()
-    predictor = EpcPrediction()
 
-    # Classification: name -> [hits, applicable-total], populated from whatever
-    # components compare_prediction reports (insertion order preserved). A None
-    # hit (the actual lodges no value) is excluded from the denominator.
-    categoricals: dict[str, list[int]] = {}
-    floor_res: list[float] = []
-    window_count_res: list[int] = []
-    window_area_res: list[float] = []
-    parts_res: list[int] = []
-    door_res: list[int] = []
-    # End-to-end (calculator-FLOORED) vs API-lodged — secondary guard, ADR-0030.
-    sap_vs_lodged: list[float] = []
-    co2_vs_lodged: list[float] = []
-    pei_vs_lodged: list[float] = []
-    # Attribution readout: how far the calculator alone is from lodged on the
-    # ACTUAL components — the floor the end-to-end numbers can reach.
-    sap_calc_actual_vs_lodged: list[float] = []
-    predicted_n = skipped_non_102 = skipped_no_cohort = 0
-
-    for postcode, certs in index.items():
-        cohort = _load_cohort(postcode, certs)
-        targets = _ground_truth_properties(cohort)
-        if len(targets) < 2:
-            skipped_no_cohort += len(targets)
-            continue
-        for held_out in targets:
-            # Only SAP 10.2 certs are valid validation targets (ADR-0030); the
-            # source cohort (`others`) keeps every vintage.
-            if held_out.epc.sap_version != _SAP_10_2:
-                skipped_non_102 += 1
-                continue
-            # Exclude every cert of the held-out address (not just the held cert)
-            # so a re-lodgement of the same property cannot leak into the cohort.
-            others = [
-                c
-                for c in cohort
-                if c.address is None or c.address != held_out.address
-            ]
-            actual = held_out.epc
-            target = PredictionTarget(
-                postcode=postcode,
-                property_type=actual.property_type or "",
-                built_form=actual.built_form,
-            )
-            comparables = select_comparables(target, others)
-            if not comparables.members:
-                continue
-            predicted = predictor.predict(target, comparables)
-            predicted_n += 1
-
-            cmp = compare_prediction(predicted, actual)
-            for name, hit in cmp.categorical_hits.items():
-                _tally(categoricals.setdefault(name, [0, 0]), hit)
-            floor_res.append(cmp.floor_area_residual)
-            window_count_res.append(cmp.window_count_residual)
-            window_area_res.append(cmp.total_window_area_residual)
-            parts_res.append(cmp.building_parts_residual)
-            door_res.append(cmp.door_count_residual)
-
-            pred_result = _result(calculator, predicted)
-            actual_result = _result(calculator, actual)
-            lodged_sap = actual.energy_rating_current
-            lodged_co2 = actual.co2_emissions_current
-            lodged_pei = actual.energy_consumption_current
-            if pred_result is not None:
-                if lodged_sap is not None:
-                    sap_vs_lodged.append(
-                        abs(pred_result.sap_score_continuous - lodged_sap)
-                    )
-                if lodged_co2 is not None:
-                    co2_vs_lodged.append(
-                        abs(_co2_tonnes(pred_result) - lodged_co2)
-                    )
-                if lodged_pei is not None:
-                    pei_vs_lodged.append(
-                        abs(pred_result.primary_energy_kwh_per_m2 - lodged_pei)
-                    )
-            if actual_result is not None and lodged_sap is not None:
-                sap_calc_actual_vs_lodged.append(
-                    abs(actual_result.sap_score_continuous - lodged_sap)
-                )
+    # PRIMARY signal — Component Accuracy, calculator-free (the shared scorer).
+    accuracy = evaluate_component_accuracy(cohorts)
 
     print(f"corpus: {CORPUS}")
-    print(
-        f"predicted {predicted_n} SAP-10.2 held-out targets "
-        f"({skipped_non_102} non-10.2 targets skipped, "
-        f"{skipped_no_cohort} had no cohort)\n"
-    )
+    print(f"predicted {accuracy.targets} SAP-10.2 held-out targets\n")
     print("--- Component Accuracy (PRIMARY, calculator-independent) ---")
-    for name, (hits, total) in categoricals.items():
+    for name, (hits, total) in accuracy.classification.items():
         if total:
             print(f"CLASSIFICATION  {name}: {hits}/{total} = {hits / total:.1%}")
     print()
-    _residual("floor_area (m2)", floor_res)
-    _residual("window_count", [float(x) for x in window_count_res])
-    _residual("total_window_area (m2)", window_area_res)
-    _residual("building_parts", [float(x) for x in parts_res])
-    _residual("door_count", [float(x) for x in door_res])
+    _residual("floor_area (m2)", accuracy.residuals.get("floor_area", []))
+    _residual("window_count", accuracy.residuals.get("window_count", []))
+    _residual(
+        "total_window_area (m2)", accuracy.residuals.get("total_window_area", [])
+    )
+    _residual("building_parts", accuracy.residuals.get("building_parts", []))
+    _residual("door_count", accuracy.residuals.get("door_count", []))
+
+    # SECONDARY guard — end-to-end vs API-lodged, calculator-FLOORED. Re-walks the
+    # same held-out targets (one orchestration via iter_predictions).
+    sap_vs_lodged: list[float] = []
+    co2_vs_lodged: list[float] = []
+    pei_vs_lodged: list[float] = []
+    sap_calc_actual_vs_lodged: list[float] = []  # the floor the end-to-end reaches
+    for predicted, actual in iter_predictions(cohorts):
+        pred_result = _result(calculator, predicted)
+        actual_result = _result(calculator, actual)
+        lodged_sap = actual.energy_rating_current
+        lodged_co2 = actual.co2_emissions_current
+        lodged_pei = actual.energy_consumption_current
+        if pred_result is not None:
+            if lodged_sap is not None:
+                sap_vs_lodged.append(
+                    abs(pred_result.sap_score_continuous - lodged_sap)
+                )
+            if lodged_co2 is not None:
+                co2_vs_lodged.append(abs(_co2_tonnes(pred_result) - lodged_co2))
+            if lodged_pei is not None:
+                pei_vs_lodged.append(
+                    abs(pred_result.primary_energy_kwh_per_m2 - lodged_pei)
+                )
+        if actual_result is not None and lodged_sap is not None:
+            sap_calc_actual_vs_lodged.append(
+                abs(actual_result.sap_score_continuous - lodged_sap)
+            )
+
     print()
     print("--- End-to-end vs API-lodged (SECONDARY, calculator-FLOORED) ---")
     _sap_line("SAP |pred − lodged|", sap_vs_lodged)
@@ -236,15 +114,6 @@ def main() -> None:
     _sap_line("  floor: SAP |calc(actual) − lodged|", sap_calc_actual_vs_lodged)
 
 
-def _tally(counter: list[int], hit: Optional[bool]) -> None:
-    """Record one classification outcome: a None hit (actual absent) is not
-    applicable and skipped; else increment the applicable total and the hits."""
-    if hit is None:
-        return
-    counter[1] += 1
-    counter[0] += int(hit)
-
-
 def _residual(label: str, values: list[float]) -> None:
     if not values:
         print(f"RESIDUAL  {label}: (none)")
diff --git a/tests/domain/epc_prediction/test_validation.py b/tests/domain/epc_prediction/test_validation.py
new file mode 100644
index 00000000..acf49a47
--- /dev/null
+++ b/tests/domain/epc_prediction/test_validation.py
@@ -0,0 +1,123 @@
+"""Behaviour of the Component Accuracy leave-one-out scorer (ADR-0030): given
+loaded postcode cohorts, hold out each SAP 10.2 target, predict it from its
+all-vintage neighbours, and aggregate the per-component hits + residuals. Pure
+(no IO, no calculator) — corpus loading is the caller's job.
+"""
+
+from datetime import date
+from typing import Optional, Union
+
+from datatypes.epc.domain.epc_property_data import (
+    EpcPropertyData,
+    MainHeatingDetail,
+    SapBuildingPart,
+    SapEnergySource,
+    SapFloorDimension,
+    SapHeating,
+)
+from domain.epc_prediction.comparable_properties import Comparable
+from domain.epc_prediction.validation import evaluate_component_accuracy
+
+
+def _comparable(
+    *,
+    certificate_number: str,
+    address: str,
+    sap_version: float,
+    wall_construction: Union[int, str] = 1,
+    registration_date: Optional[date] = None,
+) -> Comparable:
+    """A Comparable carrying a fully-populated opaque EpcPropertyData — every
+    field the predictor + comparison read (the partial-instance idiom)."""
+    epc: EpcPropertyData = object.__new__(EpcPropertyData)
+    epc.sap_version = sap_version
+    epc.postcode = "LS6 1AA"
+    epc.property_type = "2"
+    epc.built_form = "4"
+    epc.total_floor_area_m2 = 80.0
+    epc.door_count = 2
+    epc.solar_water_heating = False
+    epc.has_hot_water_cylinder = True
+    part: SapBuildingPart = object.__new__(SapBuildingPart)
+    part.wall_construction = wall_construction
+    part.wall_insulation_type = 1
+    part.construction_age_band = "K"
+    part.roof_construction = 1
+    part.roof_insulation_thickness = 100
+    part.sap_room_in_roof = None
+    floor_dim: SapFloorDimension = object.__new__(SapFloorDimension)
+    floor_dim.floor_construction = 1
+    floor_dim.floor_insulation = 1
+    part.sap_floor_dimensions = [floor_dim]
+    epc.sap_building_parts = [part]
+    epc.sap_windows = []
+    detail: MainHeatingDetail = object.__new__(MainHeatingDetail)
+    detail.main_fuel_type = 20
+    detail.main_heating_category = 2
+    detail.main_heating_control = 2100
+    heating: SapHeating = object.__new__(SapHeating)
+    heating.main_heating_details = [detail]
+    heating.water_heating_fuel = 20
+    heating.water_heating_code = 901
+    heating.cylinder_insulation_type = 1
+    heating.secondary_heating_type = None
+    epc.sap_heating = heating
+    energy: SapEnergySource = object.__new__(SapEnergySource)
+    energy.photovoltaic_supply = None
+    energy.photovoltaic_arrays = None
+    epc.sap_energy_source = energy
+    return Comparable(
+        epc=epc,
+        certificate_number=certificate_number,
+        address=address,
+        registration_date=registration_date,
+    )
+
+
+def test_scores_only_sap_10_2_targets() -> None:
+    # Arrange — a cohort of two distinct addresses: one SAP 10.2, one older
+    # (SAP 9.94). Only the 10.2 cert is a valid held-out target; the older one
+    # is kept as source evidence (its components are still valid).
+    cohort = [
+        _comparable(
+            certificate_number="A", address="1 THE ROW", sap_version=10.2
+        ),
+        _comparable(
+            certificate_number="B", address="2 THE ROW", sap_version=9.94
+        ),
+    ]
+
+    # Act
+    accuracy = evaluate_component_accuracy([cohort])
+
+    # Assert — exactly one target scored (the 10.2 cert), predicted from the
+    # older neighbour; the older cert was never held out.
+    assert accuracy.targets == 1
+    assert accuracy.rate("wall_construction") == 1.0
+
+
+def test_aggregates_a_wall_classification_miss() -> None:
+    # Arrange — the 10.2 target is solid brick (2); its only neighbour (the
+    # source) is cavity (1), so the predicted mode misses the wall.
+    cohort = [
+        _comparable(
+            certificate_number="A",
+            address="1 THE ROW",
+            sap_version=10.2,
+            wall_construction=2,
+        ),
+        _comparable(
+            certificate_number="B",
+            address="2 THE ROW",
+            sap_version=10.2,
+            wall_construction=1,
+        ),
+    ]
+
+    # Act
+    accuracy = evaluate_component_accuracy([cohort])
+
+    # Assert — both are 10.2 targets, and each is predicted from the other (the
+    # opposite wall), so wall_construction is missed both times.
+    assert accuracy.targets == 2
+    assert accuracy.rate("wall_construction") == 0.0