IBM team did 40% fewer defects than non-TDD team, Microsoft team: 60–90% (fewer defects than non-TDD fellows).
You can read more here: https://medium.com/crowdbotics/tdd-roi-is-test-driven-develo...
> The experiment was conducted on “live” projects that the teams were developing at that moment.
There are plenty of experiments on those conditions. If they had consistent results, we could trust those, but they are all over the place, what means that confounding factors are more relevant than the thing being studied.
I think the debate is not whether testing in general reduces defects (which is rather obvious), but if adhering to a stringent Test-Driven Development (TDD) process is better than regular testing (typically adding the tests after writing the code, instead of before).
The usual wisdom is that the later a problem is detected, the more it costs. 15-33% extra time for removing ~half defects very early is a great trade-off!