Microsoft
A study conducted by Microsoft found that "code quality" increased by
200% when TDD was used, compared to similar projects that didn't use
TDD.
Bhat, Thirumalesh, and Nachiappan Nagappan. "Evaluating the efficacy of
test-driven development: industrial case studies." ISESE '06:
Proceedings of the 2006 ACM/IEEE international symposium on Empirical
software engineering, 2006, pp. 356-363. DOI: 10.1145/1159733.1159787.
Microsoft Research
The results of the case studies indicate that the pre-release defect
density of the four products decreased between 40% and 90% relative
to similar projects that did not use the TDD practice.
Nagappan, N., Maximilien, E.M., Bhat, T. et al. Realizing quality
improvement through test driven development: results and experiences
of four industrial teams. Empir Software Eng 13, 289-302 (2008).
https://doi.org/10.1007/s10664-008-9062-z
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IBM
A team at IBM adopted TDD and reported a 50% decrease in the bug
defects compared to similar a similar system that was built using an
ad-hoc unit testing approach.
E. M. Maximilien and L. Williams, "Assessing test-driven development
at IBM," 25th International Conference on Software Engineering,
2003. Proceedings., Portland, OR, USA, 2003, pp. 564-569, doi:
10.1109/ICSE.2003.1201238.
NRC
A researcher at NRC's Institute for Information Technology observed
that the minimum quality increased linearly with the number of
programmer tests, independent of the development strategy employed
H. Erdogmus, M. Morisio and M. Torchiano, "On the effectiveness of
the test-first approach to programming," in IEEE Transactions on
Software Engineering, vol. 31, no. 3, pp. 226-237, March 2005, doi:
10.1109/TSE.2005.37.