FA-046 / Numerics / Open access

Missing measurements bias a mean · case 01

Absent measurements change the mean even though no observation was recorded.

Verified by executionVariant 1 · 3 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Missing records remain in the denominator after being removed from the numerator.

VERIFIED REPAIR

Use the same set of present observations for both numerator and denominator.

Unsuccessful approach: Filtering by truthiness also removes legitimate zero measurements.

Case contract

Use the same set of present observations for both numerator and denominator.

Why this case matters

A compact reasoning and repair task with executable evidence.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(values):
    return sum(x for x in values if x is not None) / len(values) if values else None
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('missing and zero', solve([None, 0, 2*N]), N)
check('all missing', solve([None, None]), None)
check('no measurements', solve([]), None)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
missing and zero	`0.6666666666666666`	`1`	Failed
all missing	`0.0`	`None`	Failed
no measurements	`None`	`None`	Passed

SHA-256 / 7b773fcc45d5dfc955d7eb3c972a6d5de0acea4f65662967122e78159d745a54

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(values):
    present = [x for x in values if x]
    return sum(present) / len(present) if present else None
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('missing and zero', solve([None, 0, 2*N]), N)
check('all missing', solve([None, None]), None)
check('no measurements', solve([]), None)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
missing and zero	`2.0`	`1`	Failed
all missing	`None`	`None`	Passed
no measurements	`None`	`None`	Passed

SHA-256 / 5bfd2a3c39f9291f46c18c614f157a9cbe2bd86d2036795f7f8cf2c702acbe3f

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(values):
    present = [x for x in values if x is not None]
    return sum(present) / len(present) if present else None
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('missing and zero', solve([None, 0, 2*N]), N)
check('all missing', solve([None, None]), None)
check('no measurements', solve([]), None)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
missing and zero	`1.0`	`1`	Passed
all missing	`None`	`None`	Passed
no measurements	`None`	`None`	Passed

SHA-256 / 3b984da2099f96b65e1eddc34cf92cc07bd3c909635a51246b1207c1d9c8f06f

Verification & scope

This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:36:49.880640+00:00.

Case digest / 8a8e014b55285512649397e413207249f25999ca4f64072cebf460212fbb3c71