Boundary Value Analysis

Concept

Most bugs live on the edges of valid ranges. BVA focuses specifically on the minimum, maximum, and values immediately adjacent to each boundary.

Partition edge:   min-1 | min | min+1 ... max-1 | max | max+1
                    ↑      ↑      ↑         ↑      ↑      ↑
                 invalid valid  valid     valid  valid  invalid

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Standard Boundary Points

For a valid range [min, max], always test:

Point	Value	Reason
min − 1	just below lower bound	should be rejected
min	lower bound itself	should be accepted
min + 1	just above lower bound	should be accepted
max − 1	just below upper bound	should be accepted
max	upper bound itself	should be accepted
max + 1	just above upper bound	should be rejected

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Why Boundaries Break

Developers write:

if age > 18:   # wrong — excludes 18
if age >= 18:  # correct

Off-by-one errors in conditions are among the most common bugs. BVA is the fastest way to expose them.

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Example: Age Validation (18–65)

import pytest


def validate_age(age: int) -> str:
    if age < 18:
        return "too_young"
    if age > 65:
        return "too_old"
    return "valid"


@pytest.mark.parametrize("age, expected", [
    (17, "too_young"),  # min - 1
    (18, "valid"),      # min (boundary)
    (19, "valid"),      # min + 1
    (64, "valid"),      # max - 1
    (65, "valid"),      # max (boundary)
    (66, "too_old"),    # max + 1
])
def test_age_boundary_values(age: int, expected: str) -> None:
    assert validate_age(age) == expected

Six targeted tests. They catch what 50 random tests in the middle of the range never will.

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String Length Boundaries

def validate_username(name: str) -> bool:
    return 3 <= len(name) <= 20


@pytest.mark.parametrize("name, valid", [
    ("ab", False),                # 2 chars — min - 1
    ("abc", True),                # 3 chars — min
    ("abcd", True),               # 4 chars — min + 1
    ("a" * 19, True),             # 19 chars — max - 1
    ("a" * 20, True),             # 20 chars — max
    ("a" * 21, False),            # 21 chars — max + 1
])
def test_username_length_boundaries(name: str, valid: bool) -> None:
    assert validate_username(name) == valid

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BVA for Collections

When a system has a limit on items (e.g., max 5 items in a cart):

Boundary	Test
0 items	empty state
1 item	minimum meaningful
4 items	max − 1
5 items	max (limit itself)
6 items	should be rejected

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Risks

Risk	Description	Mitigation
Skipping min+1 / max-1	Only testing exact boundary	Always include adjacent values
Floating-point precision	0.1 + 0.2 ≠ 0.3 in Python	Use decimal.Decimal or epsilon comparison
Inclusive vs exclusive	Ambiguous spec ("up to 65" vs "under 65")	Clarify spec before writing tests

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BVA + EP Together

EP reduces test count by grouping similar inputs. BVA ensures boundaries between those groups are tested precisely. Always use them as a pair — EP without BVA leaves the most dangerous values untested.