Regression Testing v Recession Testing

Anne-Marie Charrett wrote a blog post that I commented on extensively. I’ve reproduced the comment here:

“Some to agree with here, and plenty to disagree with too…

1. Regression testing isn’t about finding bugs the same way as one might test new software to detect bugs (testing actually does not detect bugs, it exposes failure. Whatever.) It is about detecting unwanted changes in functionality caused by a change to software or its environment. Good regression tests are not necessarily ‘good functional tests’. They are tests that will flag up changes in behaviour – some changes will be acceptable, some won’t. A set of tests that purely achieve 80% branch coverage will probably be adequate to demonstrate functional equivalence of two versions of software with a high level of confidence – economically. They might be lousy functional tests “to detect bugs”. But that’s OK – ‘bug detection’ is a different objective.

2. Regression Testing is one of four anti-regression approaches. Impact analysis from a technical and business point of view are the two preventative approaches. Static code analysis is a rarely used regression detection approach. Fourthly…and finally … regression testing is what most organisations attempt to do. It seems to be the ‘easiest option’ and ‘least disruptive to the developers’. (Except that it isn’t easy and regression bugs are an embarrassing pain for developers). The point is one can’t consider regression testing in isolation. It is one of four weapons in our armoury (although the technical approaches require tools). It is also over relied-on and done badly (see 1 above and 3 below).

3. If Regression testing is about demonstrating functional equivalence (or not), then who should do it? The answer is clear. Developers introduce the changes. They understand or should understand the potential impact of planned changes on the code base before they proceed. Demonstrating functional equivalence is a purely technical activity. Call it checking if you must. Tools can do it very effectively and efficiently if the tests are well directed (80% branch coverage is a rule of thumb). Demonstrating functional equivalence is a purely technical activity that should be done by technicians.

Of course, what happens mostly is that developers are unable to perform accurate technical impact analyses and they don’t unit test well so they have no tests and certainly nothing automated. They may not be interested in and/or paid to do testing. So the poor old system or acceptance testers working purely from the user interface are obliged to give it their best shot. Of course, they try and re-use their documented tests or their exploratory nous to create good ones. And fail badly. Not only are tests driven from the UI point of view unlikely to cover the software that might be affected, the testers are generally uninformed of the potential impact of software changes so have no steer to choose good tests in the first place. By and large, they aren’t technical and aren’t privy to the musings of the developers, before they perform the code changes so they are pretty much in the dark.

So UI driven manual or automated regression testing is usually of low value (but high expense) *when intended to demonstrate functional equivalence*. That is not to say that UI driven testing has no value. Far from it. It is central to assessing the business impact of changes. Unwanted side effects may not be bugs in code. Unwanted side-effects are a natural outcome of the software changes requested by users. A common unwanted effect here is for example, a change in configuration in an ERP system. The users may not get what they wanted from the ‘simple change’. Ill-judged configuration changes in ERP systems designed to perform straight-through processing can have catastrophic effects. I know of one example that caused 75 man-years of manual data clean-up effort. The software worked perfectly – there was no bug. The business using the software did not understand the impact of configuration changes.

Last year I wrote four short papers on Anti-Regression Approaches (including regression testing) and I expand on the points above. You can see them here: “