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White Box vs. Black Box Software Testing

I speak to our beginner students on a regular basis and it seems as though one of the more common points of confusion relates to the difference between white box and black box testing.  As these testing concepts are critical to passing both the ISEB Foundation / ISTQB Foundation & ISEB Intermediate examinations, I thought I would briefly explain the key differences between the two.

Black box testing is focused on making sure the software application actually works as intended from the end-user’s perspective.  It aims to measure the expected output of each input.  To use a non-software example, if I flip on the light switch (input) then the lamp should turn on (output), irrespective of the quality, efficiency or condition of the underlying wiring process or lamp components.  The name black box stems from the fact that you are not concerned with seeing inside “the box.”

Conversely, white box testing aims to pull back the covers and look at the inner workings of the software application.  Simply put, whereas black box testing focuses solely on results, white box testing also looks at the underlying components to make sure the inner workings are optimized.  Using the lamp example from above, white box testing would be used to make sure the wiring is structured in the most efficient way possible, and to otherwise make sure all the individual components are working correctly and optimally.

Like most things in life, both methods have advantages and disadvantages.  Black box testing is relatively quick, simple, and easy to do, but it is less comprehensive and therefore less accurate compared to white box methods.  White box testing generally offers more stability and reusability of test cases, and is also very thorough, but it can be complex and can consume a disproportionate amount of resource hours.

If you are wondering which methodology is better, it really depends on the nature of your organization, employees and industry.  When in doubt, experts usually recommend starting off with black box testing because it is easier to master, focuses on the end-user, and does not consume tons of resources out of the gate.  Then organizations can proceed to white box methods once they have mastered the basics of the black box methods.


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