Proof that TDD slows projects down

A frequent argument between people who practice Test-Driven Development (TDD) and those who don’t is whether the overhead of all those extra tests slows a project down or speeds it up. Now there’s a study to cite.

Exploding Software Myths, an article from Microsoft Research, summarizes some recent research in software development, including a study, Realizing quality improvement through test driven development: results and experiences of four industrial teams, which compares development done using TDD to “normal” projects, using data gathered from projects at Microsoft and IBM. The upshot?

“What the research team found was that the TDD teams produced code that was 60 to 90 percent better in terms of defect density than non-TDD teams. They also discovered that TDD teams took longer to complete their projects—15 to 35 percent longer.”

So there it is. TDD is slower.

Now consider an alternate story, told using the same numbers:

By doing normal development instead of TDD, teams can complete projects 14 to 26 percent faster at the expense of defect densities that are 150 to 900 percent higher.