The cost of Test Driven Development

(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at:, or follow me on Twitter.


“The nice thing about working at Microsoft,” Nagappan says, “is that the development organization is large enough that we could select teams that allowed for an apples-to-apples comparison. We picked three development projects under the same senior manager and looked at teams that used TDD and those that didn’t. We collected data from teams working on Visual Studio, Windows, and MSN and also got data from a team at IBM, since the project was a joint study.”

The study and its results were published in a paper entitled Realizing quality improvement through test driven development: results and experiences of four industrial teams, by Nagappan and research colleagues E. Michael Maximilien of the IBM Almaden Research Center; Thirumalesh Bhat, principal software-development lead at Microsoft; and Laurie Williams of North Carolina State University. 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.

“Over a development cycle of 12 months, 35 percent is another four months, which is huge,” Nagappan says. “However, the tradeoff is that you reduce post-release maintenance costs significantly, since code quality is so much better. Again, these are decisions that managers have to make—where should they take the hit? But now, they actually have quantified data for making those decisions.”

I use assertions a lot so I’m glad to read this:

One reason why assertions have been difficult to investigate is a lack of access to large commercial programs and bug databases. Also, many large commercial applications contain significant amounts of legacy code in which there is minimal use of assertions. All of this contributes to lack of conclusive analysis.

…The team observed a definite negative correlation: more assertions and code verifications means fewer bugs. Looking behind the straight statistical evidence, they also found a contextual variable: experience. Software engineers who were able to make productive use of assertions in their code base tended to be well-trained and experienced, a factor that contributed to the end results. These factors built an empirical body of knowledge that proved the utility of assertions.

…The work also brings up another issue: What kind of action should development managers take based on these findings? The research team believes that enforcing the use of assertions would not work well; rather, there needs to be a culture of using assertions in order to produce the desired results. Nagappan and his colleagues feel there is an urgent need to promote the use of assertions and plan to collaborate with academics to teach this practice in the classroom. Having the data makes this easier.

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