Sunday, September 25, 2005

Bug Priority and Severity

Differentiate Priority and Severity. The effect of a bug on the software does not automatically correlate with the priority for fixing it. A severe bug that crashes the software only once in a blue moon for 1% of the users is lower priority than a mishandled error condition resulting in the need to re-enter a portion of the input for every user every time.


Track priority and severity separately, then triage appropriately. It helps to have input from others on the team on priority. The importance of a bug is a project decision, different from the bug's perception by the Customer. In some cases it makes sense to track Urgency, the customer's point of view, separately.

Microsoft uses a four-point scale to describe severity of bugs. Severity 1 is a crash or anything that loses persistent data , i.e., messing up your files on disk. Sev 2 is a feature that doesn't work. Sev 3 is an aspect of a feature that doesn't work. Sev 4 is for purely cosmetic problems, misspellings in dialogs, redraw issues, etc. This system works very well. (Interestingly, sev 4 bugs end up getting set to priority 1 fairly often, because they are frequently very annoying to users, and fixing them is generally easy and doesn't destabilize things.)

Keep clear the difference between severity and priority". The example given says "a start-up splash screen with your company logo backwards and the name misspelled is purely a cosmetic problem. However, most companies would treat it as a high-priority bug."

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