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stack/tests/lead/test_express_stop_loss.py
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feat(lead): LEAD PY2027 financial methodology — settlement and benchmark engines
Ports the public CMS "LEAD Model PY 2027 Alignment and Financial
Methodology Paper" v1.2 (bib E4PEU3HV) as generic engines: parameters
come from the paper, everything CMS computes per ACO or per year comes
from the caller. No participant data, no organization-specific rules.

- lead.params: Table 5.2.1 discounts, 3% Quality Withhold, 2% retention
  incentive, Box 5.3.3 Stop-Loss bands, Tables 5.4.1/5.4.3 corridors, 2%
  sequestration, base-year weights, Table 3.5.1 ACPT guardrails,
  risk-score growth caps, REA/PSA shares and caps, voluntary guardrail
  and 500 minimum, Table 2.6.1 base-year alignment minimums.
- lead.settlement: Final Financial Settlement, Table 5.6.1 line by line
  (settle(SettlementInputs) -> SettlementLines).
- lead.stop_loss: Box 5.3.3 charge, banded payout, neutrality factor,
  application to PY expenditure.
- lead.monies_owed: Table 5.5.1 Total Monies Owed, APO reconciliation.
- lead.timing: run-out end, settlement-basis rule, settlement-stage inputs.
- lead.benchmark: §3 formulas — risk standardization, two-/three-way
  trend, base-year weights and sufficiency, voluntary guardrail,
  regional FFS, spending designation, REA, PSA, risk-score cap, hybrid
  alignment dollars, category blend.
- lead.express: the same as narwhals Exprs; express.settlement.settle
  settles a frame of scenarios on any backend.

Rounding: every dollar line rounds to the cent (half away from zero) as
it is produced; PBPMs, rates and factors never do. It is the only rule
under which Tables 3.3.3, 3.5.3.2, 3.5.4, 5.5.1 and 5.6.1 all reproduce
(3.5.4 line 24 sums cent-rounded cells; 3.3.3 carries unrounded PBPMs).
The Expr cents snaps its ÷100 with round(2): polars 2.0 divides a column
by a literal as a reciprocal multiply (k × 0.01), one ulp off k/100 for
~15% of values.

Missing inputs stay missing (None/null or a refused input model); the
only zeros are the paper's own (Professional "N/A" discount, no
Stop-Loss election, sequestration on losses, ineligible REA/PSA).

Paper inconsistencies recorded in tests: Table 5.5.1 line 2 is a cent
below Table 5.6.1 line 28 (.43 vs .44); Box 3.3.2 prints 1.104 then says
1.103; §3.5.1 prose widens the ACPT lower bound a year later than
Table 3.5.1 (the table is followed).

Tests: 283 in tests/lead — every published line to the cent, Expr
builders on polars and DuckDB, bit-for-bit scalar/frame parity over 160
scenarios; 100% of src/lead.
2026-10-27 14:11:10 -04:00

88 lines
2.6 KiB
Python

"""lead.express.stop_loss: beneficiary-grain Stop-Loss on polars and DuckDB,
matching Box 5.3.3 and the scalar engine."""
from __future__ import annotations
import random
import polars as pl
from lead.express.stop_loss import (
band_status,
distance_to_attachment_point,
predicted_expenditure,
residual_expenditure,
stop_loss_payout,
)
from lead.stop_loss import band_status as scalar_band_status
from lead.stop_loss import stop_loss_payout as scalar_payout
from tests.lead.paper import STOP_LOSS_PAYOUT
SCHEMA = {"actual": pl.Float64, "predicted": pl.Float64, "ap": pl.Float64}
def test_box_5_3_3_illustration(backend):
data = [
{
"actual": STOP_LOSS_PAYOUT["actual"],
"predicted": STOP_LOSS_PAYOUT["predicted"],
"ap": STOP_LOSS_PAYOUT["attachment_point"],
}
]
residual = residual_expenditure("actual", "predicted")
(result,) = backend.select(
data,
SCHEMA,
residual=residual,
payout=stop_loss_payout(residual, "ap"),
status=band_status(residual, "ap"),
distance=distance_to_attachment_point("ap", residual),
)
assert result == {
"residual": 400_000.0,
"payout": 220_000.0,
"status": "band_2",
"distance": -250_000.0,
}
def test_predicted_expenditure(backend):
data = {"regional": [1_000.0], "risk": [1.2], "months": [12.0]}
(result,) = backend.select(
data, predicted=predicted_expenditure("regional", "risk", "months")
)
assert result["predicted"] == 14_400.0
def test_payout_and_band_match_the_scalar(backend):
rng = random.Random(3)
data = [
{
"actual": rng.uniform(0, 900_000),
"predicted": rng.uniform(0, 200_000),
"ap": rng.choice([150_000.0, 98_765.43]),
}
for _ in range(200)
]
data += [
{"actual": None, "predicted": 1.0, "ap": 150_000.0},
{"actual": 1.0, "predicted": 0.0, "ap": None},
]
residual = residual_expenditure("actual", "predicted")
rows = backend.select(
data,
SCHEMA,
residual=residual,
payout=stop_loss_payout(residual, "ap"),
status=band_status(residual, "ap"),
)
for inputs, result in zip(data, rows, strict=True):
if inputs["actual"] is None or inputs["ap"] is None:
assert (result["payout"], result["status"]) == (None, None)
continue
assert (
result["payout"]
== scalar_payout(result["residual"], inputs["ap"]).total_payout
)
assert result["status"] == scalar_band_status(result["residual"], inputs["ap"])