What is the difference between bug severity and bug priority?
This is a basic question. Most projects mix both priority and severity into a single value. What reasons are there to separately assign severity and priority to a defect? What do testing methodologies say about that? It is practical to choose different labels for severity and different for priority?
The severity of a bug is a measure of how important the bug is to the end user: how much it breaks, how badly it breaks things, how difficult it is to get work done with this bug in place. The priority of the bug is a measure of how important the bug is to the development team. This will take into effect the severity, but also the development time and effort, and how much impact fixing it would have on the rest of the product. Something that is severe and easy to fix may be higher priority than something that is severe and difficult to fix; some teams will prioritize "low-hanging fruit", things that are easy to fix, over more complex fixes regardless of severity. There can be complex rules around prioritization of bugs, balancing the factors involved; therefore, usually prioritization is done by a project manager or as part of a Scrum meeting or some such. Severity can be determined when filing the bug, in isolation of any knowledge of the development effort.