Boundary Value Analysis & Equivalence Class Testing (Functional Testing)

Both are black-box (functional) testing techniques that design test cases from the input specifications without looking at internal code.

Equivalence Partitioning (Equivalence Class Testing)

Idea: Partition the input domain into classes such that all values in one class are expected to be treated identically by the program (same output/behaviour). Then test one representative value from each class — this drastically reduces the number of test cases while keeping high coverage.

Rules for partitioning:

• Valid equivalence classes — inputs the system should accept.
• Invalid equivalence classes — inputs the system must reject.

Example: A field accepts integers 1–100.

• Valid class: {1 ≤ x ≤ 100} → test one value, e.g., 50.
• Invalid class 1: {x < 1} → test, e.g., 0 (or −5).
• Invalid class 2: {x > 100} → test, e.g., 200.
• (Non-integers form further invalid classes.)

Benefit: systematic coverage of the whole input domain with minimal test cases; it reveals the presence of "class-wide" defects (a whole category of inputs mishandled).

Boundary Value Analysis (BVA)

Idea: Experience shows errors concentrate at the boundaries of input ranges (off-by-one mistakes are the most common defect). BVA therefore tests the values on and immediately around each boundary: minimum, just-above-minimum, just-below-maximum, maximum (and, depending on the flavour, also nominal interior values).

For the range [1, 100]:

• Test 1, 2 (lower boundary + just above),
• Test 99, 100 (just below max + max),
• Optionally 0 and 101 (just outside, invalid).

Benefit: catches off-by-one and boundary-handling errors that equivalence partitioning alone would miss; extremely cost-effective because most real defects sit at boundaries.

How they help

• High fault-detection per test case — both techniques target where defects actually cluster (representative classes + edges).
• Reduced test count — instead of exhaustive input testing, a few well-chosen values cover the domain.
• Complementary — equivalence partitioning provides representative coverage inside the classes; BVA adds coverage at the edges, so used together they give strong functional coverage.

Example summary (input 1–100)

Technique	Test values
Equivalence classes	50 (valid), 0 & 101 (invalid)
BVA	1, 2, 99, 100 (and 0, 101 as boundaries)