ITPEC IP April 2018 Question 46

Check normal cases and the intended behaviour on input errors — a test must also show that wrong input is handled the way the specification says.

Test cases come from valid data and from invalid data: out-of-range values, wrong types, missing items and boundary values. The specification defines what should happen with bad input, such as an error message or rejection, and the test confirms it. A program that works only for valid input will fail in operation.

Answer (d)

Why not others:

• •(a) — reversed: a black box test checks the external specifications (inputs against outputs), and a white box test checks the internal structure (paths and branches)
• •(b) — the opposite holds: where many errors have been found, more tend to remain, which is why errors found are tracked on a reliability growth curve
• •(c) — testing can show that errors exist but cannot prove there are none; a test plan sets completion criteria rather than assuming every error will be found

Key rule: Build test cases from both valid and invalid classes of input plus their boundaries; the purpose of testing is to find errors, not to prove their absence.

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