"This is where proven practices step in, and there are a few of them. Test-driven development has been shown to reduce defect rates 40% to 90%, with an increase in development time in the 15%-35% range. Code reviews have also been shown to decrease defect rates, in some cases up to 80% over manual testing."
Now, in the same book where we report on that, there's a meta-study compiling all of the evidence that we had in 2010 about test-driven development. And it turns out that on balance, there is no evidence that it has any impact up or down on the quality of software or the speed with which it's produced.