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Difference between defect and bug in testing

In computer science technical writing, especially in software engineering, what's the difference between fault, error and defect?


To quote the Software Engineering Body of Knowledge




Failure: “The [incorrect] result of a fault”

Mistake: “A human action that produces an incorrect result”

Based on my understanding of above definition, error is the result of fault, i.e., failure. Could someone explain more clearly?

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migrated from academia.stackexchange.com Jan 22 '13 at 3:36


marked as duplicate by gnat, StuperUser, Mark Booth, Thomas Owens Jan 22 '13 at 10:59


I strongly suspect the answer is nothing, except in papers that explicitly invoke IEEE610.12-90. –  JeffE Jan 20 '13 at 22:31
In IEEE610.12-90's definitions the key word that differentiates an error from a fault is "computed". An error is an incorrect result of a completed computation, a fault is an incorrect step that prevented computation from completing (e.g. a syntax error). –  Yannis Rizos Jan 21 '13 at 4:42
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2 Answers 2

According to Fundamental Concepts of Dependability:

A system failure is an event that occurs when the delivered service deviates from correct service. A system may fail either because it does not comply with the specification, or because the specification did not adequately describe its function. An error is that part of the system state that may cause a subsequent failure: a failure occurs when an error reaches the service interface and alters the service. A fault is the adjudged or hypothesized cause of an error.

I understand defect as just another name for fault.


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With reference to the definitions used in the linked article, an error is not the result of a fault, but potentially the cause - if the error is outside of the tolerance of the system. Most systems dealing with real data have some degree of error, but are designed to tolerate them.

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