White-box Testing
White-box testing is a testing approach that designs and executes tests based on the internal structure, implementation, and code of the software.
“Testing based on an analysis of the internal structure of the component or system.”
(Definition reproduced from the ISTQB Glossary. Copyright belongs to ISTQB.)
View the complete ISTQB GlossaryWhat is White-box Testing?
Testers use knowledge of the program's logic, control flow, and data flow to verify that internal structures behave correctly and achieve adequate structural coverage.
Internal knowledge: Requires understanding of source code or design.
Structural focus: Verifies statements, branches, conditions, and paths.
Coverage driven: Uses structural coverage metrics to assess completeness.
Real World Example
A tax calculation module applies different deductions depending on income band, residency status, and submitted documents.
Reviewing the final tax amount is not enough when important code paths may never execute during testing. White-box testing helps the tester reason about the structure behind the result.
The tester works with the developer to identify decisions or paths in the calculation logic and designs inputs that exercise the meaningful alternatives.
The team gains better evidence about hidden logic because White-box testing connects tests to the implementation structure where certain defects can hide.
Practice Questions
Question 1
Which statement BEST describes White-box testing in the context of ISTQB terminology?
Question 2
A tester needs to explain White-box testing to a non‑technical stakeholder. Which approach is MOST appropriate?
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