Think of the full software development life cycle. We gather requirements, create user stories, break them down into tasks, estimate, fight with your client about how long it’s all going to take, renegotiate the timings (what’s 35% of the development time on unit tests? Do we need those?), submit a revised timescale with cut down requirements and no time allocated to testing. Then we start development.

And soon the problems start coming up. At the end of the first sprint the client raises a few bugs with the software delivery… We didn’t account for those in the plan! And we can’t estimate them – we don’t know the size of the problem. That’s why it’s a bug… So we allocate some time in the next sprint to work on the top priority bugs and commit to developing fewer features.

The second sprint finishes, and more defects are raised in the software – some of them can be attributed to requirements not been defined clearly enough, but some of them are just code that we don’t understand how it even worked in development! Did someone actually test it before it went into the main development branch?! (how many people can honestly say they have never heard that one before…?)

Pretty soon you’re way behind the plan. The number of bugs that we cannot estimate and didn’t plan for are sucking up too much time and we don’t know how long it’s going to take to get back to a point where we know how long the project is going to take.

So what’s to be done? Bugs throughout the development life cycle are inevitable – but because we cannot know how many there will be and how long they will take to fix, we need to be able to minimise the number that crop up.

Some research done by microsoft ( has shown that spending 15-35% extra development time on adopting test driven development resulted in a pre-release defect density decrease of 40-90%.

The ideal is when we can plan, develop and release software that contains no defects in a timescale that everyone agrees on. This wont happen any time soon – but an increase of 15-35% development time for up to a 90% decrease in the number of defects has *got* to be worth it by anybody’s standards.

Have you used TDD (or BDD) and experienced similar results? Comments encouraged!