There are 3 main areas of interest in the discussion of benefits of static vs dynamic typing.

- Quality (How many bugs)

- Dev time (How fast to develop)

- Maintainability (how easy to maintain and adapt for years, by others than the authors)

The argument is often that there is no formal evidence for static typing one way or the other. Proponents of dynamic typing often argue that Quality is not demonstrably worse, while dev time is shorter. Few of these formal studies however look at software in the longer perspective (10-20 years). They look at simple defect rates and development hours.

So too much focus is spent on the first two (which might not even be two separate items as the quality is certainly related to development speed and time to ship). But in my experience those two factors aren't even important compared to the third. For any code base that isn't a throwaway like a one-off script or similar, say 10 or 20 years maintenance, then the ability to maintain/change/refactor/adapt the code far outweigh the other factors. My own experience says it's much (much) easier to make quick and large scale refactorings in static code bases than dynamic ones. I doubt there will ever be any formal evidence of this, because you can't make good experiments with those time frames.