and priority to come up with a composite risk number. While this intuitively sounds like a way to resolve the priority-severity divide, I suggest using such an approach with extreme caution. It's multiplying apples by oranges in an attempt to quantify bananas. Risk is yet a third type of information.
The risk associated with any bug depends on the severity of the issue, certainly. But it also depends on the likelihood that the user will run into it as well as the possible losses that might occur. I don't attempt to quantify all this when assessing the severity of an issue. In fact, I think that in most cases assessing the risk of a single issue takes more time than it's worth. Only for potentially poisonous bugs involving dangerous fixes do I really want to weigh the risk of fixing it against the risk of not fixing it.
Establish Work Precedence
The best way to avoid confusion about what comes first is to ensure everyone in the organization takes their cues for work precedence from priority and nowhere else. Developers fix P1 defects first. Testers verify P1 fixes first. Technical writers document P1 issues first. Everyone works in priority order: the priority reflects importance to the business. Saying, "This bug is more severe than that one so I'll work on it first" is as bad as saying, "I like this bug more, so I'll work on it first." The severity rating is technical information used by managers as a piece of the formula in determining the priority rating. The priority rating is the final word on the order in which the work is done by programmers, testers, and everyone else.
The ultimate lesson here, regardless of the terms or levels you use to categorize your bugs, is that any classification scheme will only be effective if everyone agrees on definitions. So perhaps that's the very first question to ask when an argument is brewing about severity, priority, or risk: "Help me understand exactly what information you're using from each defect record and how you're using it?"
Clarify Your Ranking for System Problem Reports , By Johanna Rothman