Thinking outside the Boxes


We've all heard the terms "black box" and "white box" thrown around in a variety of contexts, but what do they really mean? Industry expert Bret Pettichord explains some of the fundamental problems with using these terms as shorthand labels for techniques and how they can lead to confusion.

As a writer on software testing, I get questions from testers who need help. Twice in the past week people studying for software testing certification exams have written to me to get clarification about a column I wrote awhile back for StickyMinds: "Five Ways to Think about Black Box Testing." I wrote that column after I noticed that several people on my project were using the term "black box testing" in fairly different ways. Recognizing a pattern in the various interpretations, I came up with even more definitions of black box testing that I considered to be just as valid. So I wrote the column for StickyMinds, and I revised the test plan for my project to be specific about how those terms were being used. That resolved the issue for my team, but how does it help someone who is looking for a definitive answer for an exam?

Fundamentally, the problem with terms like "black box" and "white box" is that they describe the thinking behind the tests, not the tests themselves. They describe what kind of information was or wasn't used in the design of the tests. Black box means we don't know what the code looks like when we write out tests. White box means we do. (Though logically, it should really be called "clear box.") So you can't tell by looking at the test itself whether it is a black box test or a white box test. You'd have to know how it was created.

To add to the confusion, both terms are commonly used—even by experts—as shorthand labels for techniques. Code coverage analysis is typically described as a white box technique, because it provides a method for assessing tests based on how they exercise (or cover) the code. This requires knowing what the code looks like. On the other hand, boundary value analysis is typically described as a black box technique, because it starts by noticing boundaries in external program behavior and then suggests focusing testing at those boundaries. It doesn't require knowledge of the internals of the code.

But code coverage tools can be used with black box tests. What would you call that? And boundary value analysis can benefit from knowing where internal boundaries are in the code. Is this gray box testing?

Black box and white box are simply metaphors. They can be useful to highlight an important issue in testing: How much knowledge of the system do we need before we can apply a particular technique? But that's just be the beginning of thinking of how to test software. The black box metaphor shouldn't be seen as implying that some testing is better when done out of ignorance. And both metaphors could be criticized for orienting to the code, rather than the customer. Sadly, to invert the metaphor, many programmers and development organizations treat the customers and users as black boxes, when they might be better off trying to understand the internal goals and thought processes that their users have when they are using the software.

Any metaphor can be either useful or misleading depending on how it is used. Black box and white box become particularly dangerous when they are mistaken for scientific terminology. Consider these questions from the preparation workbook for the QAI certified software test engineer exam that the testers asked me about


About the author

Bret Pettichord's picture Bret Pettichord

Bret Pettichord is an independent consultant specializing in software testing and test automation. He co-authored Lessons Learned in Software Testing with Cem Kaner and James Bach and edits He is currently researching practices for agile testing. Contact him at or

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