Object Mentor Blog: TDD Derangement Syndrome

TDD Derangement Syndrome

Uncle Bob — Wed, 07 Oct 2009 08:32:00 -0500

My recent blog about TDD, Design Patterns, Concurrency, and Sudoku seemed to draw the ire of a few vocal TDD detractors. Some of these people were rude, insulting, derisive, dismissive, and immature. Well, Halloween is not too far away.

In spite of their self-righteous snickering they did ask a few reasonable questions. To be fair I thought it would be appropriate for me to answer them.

Is there any research on TDD?

It turns out that there is a fair bit.

One simple google search led me to this blog by Phil Haack in which he reviewed a TDD research paper. Quoting from the paper:

We found that test-first students on average wrote more tests and, in turn, students who wrote more tests tended to be more productive. We also observed that the minimum quality increased linearly with the number of programmer tests, independent of the development strategy employed.

The same google search led me to this blog by Matt Hawley, in which he reviewed several other research papers. Part of his summary:

* 87.5% of developers reported better requirements understanding. * 95.8% of developers reported reduced debugging efforts. * 78% of developers reported TDD improved overall productivity. * 50% of developers found that it decreased overall development time. * 92% of developers felt that TDD yielded high-quality code. * 79% of developers believed TDD promoted simpler design.

Actually, I recognize some of Matt’s results as coming from a rather famous 2003 study (also in the list of google results) by Laurie Wiliams and Boby George. This study describes a controlled experiment that they conducted in three different companies. Though Matt’s summary above is based (in part) on that study, there is more to say.

In the George-William study teams that practiced TDD took 16% longer to claim that they were done than the teams that did not practice TDD. Apparently tests are more accurate than claims since the non-TDD teams failed to pass one third of the researcher’s hidden acceptance tests, whereas the TDD teams passed about 6 out of 7. To paraphrase Kent Beck: “If it doesn’t have to work, I can get it done a lot faster!”

Another point of interest in this study is that the TDD teams produced a suite of automated tests with very high test coverage (close to 100% in most cases) whereas most of the non-TDD teams did not produce such a suite; even though they had been instructed to.

Jim Shore wrote a review of yet another research summary which I found in the same google search. This one combines 7 different studies (including George-Williams). Here the results range from dramatically improved quality and productivity to no observed effect.

Finally, there is this 2008 case Study of TDD at IBM and Microsoft which shows that TDDers enjoy a defect density reduction ranging from 30% to 90% (as measured by defect tracking tools) and a productivity cost of between 15% and 35% (the subjective opinion of the managers). I refer you back to Kent Beck’s comment above.

I’m sure there is more research out there. After all this was just one google search. I think it’s odd that the TDD detractors didn’t find anything when they did their google searches.

Oh yeah, and then there was that whole issue of IEEE Software that was dedicated to papers and research on TDD.

What projects have been written with TDD, hmmm?

Quite a few, actually. The following is a list of projects that have an automated suite of unit tests with very high coverage. Those that I know for a fact use TDD, I have noted as such. The others, I can only surmise. If you know of any others, please post a comment here.

JUnit. This one is kind of obvious. JUnit was written by Kent Beck and Erich Gamma using TDD throughout. If you measure software success by sheer distribution, this particular program is wildly successful.

Fit. Written by Ward Cunningham. The progenitor of most current acceptance testing frameworks.

FitNesse. This testing framework has tens of thousands of users. It is 70,000 lines of java code, with 90%+ code coverage. TDD throughout. Very small bug-list. Again, if you measure by distribution, another raving success.

Cucumber,
Rspec. These two are Testing frameworks in Ruby. Of course you’d expect a testing framework to be written with TDD, wouldn’t you? I know these were. TDD throughout.

Limelight. A gui framework in JRUby. TDD throughout.
jfreechart.
Spring
JRuby
Smallsql
Ant
MarsProject
Log4J
Jmock

Are there others? I’m sure there are. This was just a quick web search. Again, if you know of more, please add a comment.

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TDD is a means to an end. Sometimes it’s highly appropriate, sometimes it just creates the illusion of progress while obscuring ignorance of how to actually solve the problem. It would be more useful to focus on when TDD is the right tool (and when it’s counterproductive), rather than asserting that it’s always the best technique.

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