Ask HN: What are the empirical facts about programming?

What are the facts (and sources) about programming that have been justified empirically?

For instance, we've heard that the ratio of defects to lines of code is constant irrespective of programming languages. The source of this claim is apparently in the book, Code Complete, by Steve McConnell.

What are some other facts that are similarly justified?

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There aren't many empirical facts about programming. Most of it is conjector, experience, and wisdom expressed in pseudo-mathmatical terms since that's what most programmers interact with daily.

I know. I suspect that it will be a small list, but it would still be nice to gather these facts in one place.

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