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Sunday, August 22, 2010

Bug Severity vs. Priority

Severity and priority are two ways of thinking about software bugs and deciding which ones should get fixed and in what order. Not everyone uses both. I’ve been in places where bugs are categorized by only one of them. But inevitably in those places, I find that the two ways of thinking get mixed and people end up confused. As I use them, bug severity and priority are different and we need both.

Severity is about the risk a bug poses if it gets out into the wild. I’ve written about testing and software risk elsewhere. See

Briefly, software risk can be characterized as:

The potential for
some fault, failure or other unintended happening
in the implemented system
to cause harm or loss
to one or more persons or organizations.

We assess the risk of a bug by asking questions about impact and probability. How much harm could this bug cause to some thing the customer cares about, such as human safety or the bottom line? How likely is this bug to manifest and how likely is that harm to occur if it does?

If a bug could cause significant harm but only manifests under very unlikely circumstances, then we might decide it’s less severe than a bug that could cause less harm but manifests frequently. Or not, depending on the context.

Most organizations have standard criteria for classifying bug severity, such as:

Severity 1 – Catastrophic bug or showstopper. Causes system crash, data corruption, irreparable harm, etc.

Severity 2 – Critical bug in important function. No reasonable workaround.

Severity 3 – Major bug but has viable workaround.

Severity 4 – Minor bug with trivial impact.

Typically, Severity 1 and 2 bugs must be fixed before release, where 3’s and 4’s might not be, depending on how many we have and on plans for their subsequent disposition.

Priority, on the other hand, is about the order in which bugs need to be fixed. Often, priority and severity run hand-in-hand: a bug is both high severity and high priority to fix. But that’s not always true. Occasionally in testing we’d like to have lower-severity bugs fixed so we can explore an area more thoroughly and see if they’re masking something else. Particularly on large projects, we can also find that we have a larger number of high severity bugs open than the programmers can readily fix. In this case, we need to specify the order for fixes based on where we plan to test next. To some extent also, ease of programming kicks in. A programmer working on high severity bugs in a particular module may choose to fix the low severity bugs in the same session.

At least theoretically, bug severity doesn’t change. The potential for business or technical impacts stays pretty much the same throughout the development project. (The passage of time can affect risk, but that’s a subject for another post.)

Priorities for fixing bugs do change depending on where we are in the project. At first, it’s testing priorities that matter most. But the closer we get to release, the more important the customer’s priorities become, to the point where they take over entirely.

And that brings us to an essential question about both severity and priority. Who gets to decide?

Ultimately, it’s the customer’s prerogative to decide both severity and priority (using “customer” as the stand-in here for “key decision-makers”). We—testers, project managers, and programmers alike—can make educated guesses about business risk and even about business tolerance for risk, but we can’t really know and we certainly can’t decide for the customer. Similarly, we can’t decide bug fix order for the customer, who frequently has different priorities from ours.

That’s not to say we abdicate responsibility. It’s a tester’s job to try and represent the customer’s point of view when they are absent. It’s also our job to help customers make those decisions. We do that by attempting to understand the true significance of a bug and communicating our understanding. We also ask questions to help our customers assess relative risk. (As anyone who has ever supported UAT knows, customers frequently assume everything is equally high risk until we ask those questions.)

It’s good to engage with customers and ask those questions early, ideally as we go through the development project. (Agile projects have it easy in this regard.) Early engagement is especially important for assigning severity.

But until we reach the point in the project of determining bug fix priorities for release, it’s only practical for the entire development team to set priorities according to what’s needed to move the project forward. Most often that means the testers’ needs: the bug fix priorities for testing.

Of course, there are other factors affecting priority during a development project, including the relative risk and cost of fixing a bug. But generally speaking, it makes sense for testers to drive priority until we switch over to the customer’s priorities for release.