I’ve written a few articles about unit testing on this blog, but so far they’ve been about how to go about doing so and what supporting libraries/frameworks are available. Now I want to look at why unit testing is a good idea. In particular, this article will focus on the business perspective.

Many software developers embrace the unit testing of code with enthusiasm. Unfortunately though, there are many who see it as something that will just hinder them in getting their work done – after all, they will have to write significantly more code, won’t they? It is even more unfortunate that many managers see things the same way. So, why unit test, given that developers will indeed have to write significantly more code?

The fact is that, from a business perspective, unit testing earns its keep on the cost/benefit scale. The 1988 paper "Understanding and Controlling Software Costs" by Barry Boehm and Philip Papaccio states:

…the cost of fixing or reworking software is much smaller (by factors of 50 to 200) in the earlier phases of the software life cycle than in the later phases

The authors also provide references in support of this statement. Now look at the numbers: assuming the least worst case, catching an error early make it cheaper to fix by a factor of fifty. Compare this with the amount of effort needed to write a unit test – and remember this is the least worst case. It seems that unit testing is definitely earning its keep! Unfortunately, my research suggests there is a lack of recent data published on the web in this field. However, although the paper I have cited is quite old, it is hard to imagine that things have improved in the light of today’s technology. In fact, things are likely to be worse because computer systems are much more complex, as they attempt to solve much more complex problems.

Unfortunately, unit testing will not guarantee completely bug free code. Somewhere, something will get missed. It is likely, for example, that unit test coverage will miss some aspects – corner cases in particular – of the code. Despite this, the evidence suggests that it does contribute significantly.