The true meanings to Priority and Severity

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I was talking to a friend the other day about this and that going over a few ideas and stuff, when he came out with the best definition of Priority when associated with software defects. The typical definitions go along the following lines.

Priorities and Severities:
Priority is how important something is to fix, while Severity is how severe the impact is on the system as a result of the bug.

Priority:
The priority of a defect is based upon the importance and urgency of resolving the defect. Priorities are usually High (P1), Medium (P2), and Low (P3);

High (P1) – (Associated 1 with Critical (S1) and Serious (S2))
This defect must be fixed as soon as possible. Defect is blocking further progress in this area. It should be resolved immediately. (Must be fix within 24-48 hours. Blocking build test)

Medium (P2) – (Associated 1 with Serious (S2) and Moderate (S3))
This Defect should be fixed prior to the product release. It must be fixed as soon as possible. Bug is blocking further progress (testing) in this area

Low (P3) (Associated 1 with Moderate (S3) and Minor (S4))
This Defect should be fixed if time allows, it can be resolved in a future major system revision or not resolved at all.

Severity:
The severity of a defect is based upon the degree which the defect impacts the application. Severities are Critical (S1), Serious (S2), Moderate (S3) and Minor (S4).

Critical (S1) – (Associated with Priority 1)
The defect causes the system crash or loses data. This defect has no work around. Deviation between the specification and the behavior of the Application (causes the Application to function) No work around is available.

Serious (S2) – (Associated with Priority 1 & 2)
The Application will operate, but its operation is severely restricted. The defect causes major functionality or other severe problems; product crashes in obscure cases. This defect causes the system to produce incorrect, incomplete, inconsistent results or impairs the system usability. There is a work around. Deviation between the Specification and the behavior of the application (causes the Application to function improperly, i.e. incorrect totals, paint types, etc.)

Moderate (S3) – (Associated with Priority 2 & 3)
The Application will operate with limitations that are not critical to its overall operation. The defect causes minor functionality problems, may affect "fit and finish" of the product. The defect does not cause a failure, but impairs usability, and interferes in the fluent work of the program. Violation of standards, conventions, or rules that would not result in a deviation from requirements if not corrected, but could result in difficulties in terms of maintenance or clarity of purpose (workmanship)

Minor (S4) – (Associated with Priority 2 & 3)
Bug contains, formatting, typographical, spelling defect whose correct interpretation is obvious to a knowledgeable reader, or unclear wording of error messages in low visibility fields. The problem does not impact the use of the product in any substantive way. This defect affects nonfunctional aspects of the application. A work around is available. The Application can be used with only slight inconvenience.

This friend of mine put it more simply stating that a High P1 is something we have to fix or lose our customers, a Medium P2 is something that would be nice to fix but it can wait, and a Low P3 is something we will never get to so why bother. As far as Severity goes a Critical S1 is bad enough that it will cause Customer Support headaches with the increase customers calls, Serious S2 is something that the customer won’t like but they have a work around, a Moderate S3 is an issue we are betting the customer will tolerate, and a Minor S4 is something that the customer won’t even notice.

I believe he hit it on the head, how many times has a product been release with no P1s or P2s but a ton of P3s and P4s? The answer is every time a product is released. In the many years, I’ve been in the industry I have seen my friend’s definition put to use more often than not. I was working at a company which had defects 13 years old which have never been touched and the reason given for these defects being left in the product – the customer has not complained about it so it must not be a problem. Yep he has it right.
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