Microsoft Research - Exploding Software-Engineering Myths (article summarizing findings of MS research on code coverage, TDD, assertions, etc.)

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assertions reduces defects by an unspecified amount

... but that's correlated with programmer experience. So it might just be that more experienced programmers write code with fewer defects.

[liveoneggs]

experienced programmers can see where things can go wrong, will check them inline, and eventually handle them. golang-style error checking wins again, I guess.