Severity versus Priority

Many parts of testing require a pedantic view of words and the ISTQB Certified Tester syllabus is very specific on the meaning of words.

Severity is defined as the extent to which a defect can affect the software.

In order to use a common language for testing it is very useful to define the levels of severity so that when one person describes a defect as critical, we all know what they mean. There are 5 levels of severity defined as follows:

  • Critical: the defect causes the system or a component to crash resulting in that part of the system being unusable.
  • Major: the same as above, except a workaround is possible.
  • Moderate: the system does not crash but the desired results are incorrect.
  • Minor: the defect does not effect the usability of the system and the desired results are easily obtained with a workaround.
  • Cosmetic: the defect effects the look and feel of the system but does not effect the functionality.

Priority defines the order in which we should resolve a defect

This priority is set by the tester and is set based upon the customers requirements. There can be 3 priority levels as follows:

  • Low: the defect should be fixed but can be deferred until more serious defects have been fixed.
  • Medium: the defect can wait until the next build or version to be fixed.
  • High: the defect must be fixed asap. The system cannot be used until the fix is done.

I hope at this point the distinction between severity and priority are clearer. The tester has a big responsibility to record defects objectively and accurately. Having to decide on priority and severity in an incident report are made easier when the above levels are known to all and can be referred to for guidance.

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