The best type of test depends on the type of software being developed. For the sort of statistical software that I have been involved with, I think that system

The best type of test depends on the type of software being developed. For the sort of statistical software that I have been involved with, I think that system level tests (with synthetic and/or real data) give tremendous bang for the buck. This is particularly true if the data is high volume, relatively homogeneous (in some sense), and most of the top-level interfaces are fixed fairly early on. Many other projects are not like this, and so may benefit more (proportionally) from different approaches to testing.