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Software Engineering | Differences between defect, bug and failure

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Generally, when the system/application does not act as per expectation or abnormally, we call it an error or it’s a fault and so on. Many of the newbies in the Software Testing industry have confusion about using this, so let’s know what is the difference between defect, bug, error and failure. We will see these terms in detail one by one. 

What is Defect?

The bugs introduced by the programmer inside the code are called defects. The defect is defined as the deviation from the actual and expected result of an application or software in other words, defects are defined as any deviation or irregularity from the specifications mentioned in the product functional specification document. The defect is also solved by the developer in the development phase or stage.

Reasons for Defects

  • Any deviation from the customer’s requirements is called a defect.
  • Giving wrong input may lead to defects.
  • Any error in the logic code may lead to a defect.

What is Bug?

Sometimes most people are confused between defect and bug, they say that bug is the informal name of defect. Actually, bugs are faults in systems or applications that impact software functionality and performance. Usually, bugs are found in unit testing by testers.

Types of Bugs

  • Functional Errors
  • Compilation Errors
  • Missing commands
  • Run time Errors
  • Logical errors
  • Inappropriate error handling

defect, bug and failure

Defect, Bug and Failure

What is Failure?

When a defect reaches the end customer, it is called as Failure. Once the product is completed and it is delivered to the customers and if the customer find any issues in product or software then it is the condition of failure of product. In other words, if an end user finds an issue in product then that particular issue is called as failure.

Reasons of Failure

  • Human errors or mistakes may lead to failure.
  • Environmental conditions.
  • The way in which system is used.

Example of Defect

print("Addition of {} and {} = {}.".format(a, b, ans))

When you compile and run this program you see the printed statement as below:

Addition of 7 and 5=35 

This is program of adding two numbers but the output is deviated from it’s actual result which is 12. Now we have detected a failure. As the failure has been detected a defect can be raised.

Defect vs Bug vs Failure




The deviation between actual and expected result is called defect.

Fault in system which impact functionality or performance of the software.

When defect comes in user end it is called failure.

There are various types of defects like Arithmetic, Logical, Syntax, Multithreading, Interface, Performance defects etc.

There are various types of bugs like arithmetic bugs, logical bugs or resource bugs etc.

It has no type, its just the application won’t run on user end.

Usually tester identifies the defect in the code and then developer need to fix it.

Here, the test engineers raise the bug in the developers code.

It is raised or find by the manual test engineer.

It is basically caused by the developer in development phase of software.

It is caused because of various aspects like Lack of communication, Performance Errors, Lots of recycling, non existence of policy framework etc.

It is caused because of human errors or environmental conditions, system usage etc.

It can be prevented by correctly doing the software development, or by consistently reviewing the code so that defect won’t arise.

It can be prevented by step by step evaluating the code, or by providing programming language support.

It can be prevented by again verifying the requirement specification or by retesting the application basically, so that it won’t fail on user end.

Previously Asked Questions

Q.1: ____ refers to the discrepancy among a computed, observed or measured value and the true specified or theoretically correct values? [UGC NET CSE | September 2013 | Part 3]

(A) Fault

(B) Failure

(C) Defect

(D) Error

Solution: Correct Answer is (D).

Q.2: In software testing, how the error, fault and failure are related to each other? [UGC NET CSE | December 2015 | Part 2]

(A) Error leads to failure but fault is not related to error and failure

(B) Fault leads to failure but error is not related to fault and failure

(C) Error leads to fault and fault leads to failure

(D) Fault leads to error and error leads to failure

Solution: Correct Answer is (C).

Q.3: Which one of the following is not a definition of error ? [UGC NET CSE | June 2014 | Part 3]

(A) It refers to the discrepancy between a computed, observed or measured value and the true, specified or theoretically correct value.

(B) It refers to the actual output of a software and the correct output.

(C) It refers to a condition that causes a system to fail.

(D) It refers to human action that results in software containing a defect or fault.

Solution: Correct Answer is (C).

FAQs On Defect, Bug and Failure

Q.1: In which phases of software development defect, bug and failure come?


Phases in which Software Development Defect comes:

  • In Testing Phase if Testers find any mismatch they call it as Defect or Bug or Fault.
  • In Production Phase if Customers or End Users find any mismatch they call it as Failure.
  • In the Development Phase, if developers find any mismatch they call it an Error or mistake.

Q.2: What is the producible defect ?


If in every execution i.e, if every time defect came then it is called producible defect.

Q.3: How failure analysis is done in software engineering?


To analyse failure some techniques are used like Fault Tree Analysis (FTA), Failure Mode and Effects Analysis (FMEA).

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Last Updated : 31 Aug, 2023
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