Bug Classification:

Bugs are inevitable in any software development life cycle. Everybody knows that. However, you have a deadline to meet. You have a release to get over with. There comes a point in time for a release when you take a call and decide what bugs to fix and what not to fix.

Here comes the importance of bug classification. The decision on what bugs to fix and what not to fix is made based on the bug classification.

Bug classification is done primarily to separate the most important bugs from the not so important ones.

Bug severity describes how much damage a bug is doing.

Bug priority describes how important it is to fix that bug.

At first glance these two might appear the same. But they are different. They give a good picture of what to do when used together.

Consider these scenarios:
1. A bug may do a good deal of damage. But if it rarely occurs, it may be less important to fix
2. A bug may do some damage. But if it occurs consistently, it may be more important to fix

How to determine bug severity
Severity How much damage the bug does
Blocker Application or major section freezes, crashes, or fails to start. Data is corrupted.
Critical Key feature does not work, cannot be used, or returns incorrect results.
Major Key feature is difficult to use or looks terrible. A secondary feature does not work, cannot be used, or returns incorrect results
Normal Secondary feature is difficult to use or looks terrible. Minor feature does not work, cannot be used, or returns incorrect results
Minor Secondary feature has a cosmetic issue. Minor feature is difficult to use or looks bad.
Trivial Minor glitches in images, not so obvious spell mistakes, etc
Enhancement Improvement to product features due to bad usability or based on feedback from users. This is not part of product requirements / design originally.

How to set bug priority

Priority Priority description Which severity bugs
P1 Must be fixed All blockers and criticals. Majors affecting many customers.
P2 Highly desirable, scheduled, but not stop ship Majors affecting some customers. Normals affecting many customers.
P3 Nice to have, but not scheduled Normals affecting some customers. Minors and trivials that affect many customers
P4 Not likely to be fixed soon Minors and trivials affecting a few customers