Thanks for the pointer. I appreciate it. I am going to try digging up the reference.

I find it hard to believe though. Beyond the code reading challenges that developers have had, I have seen highly varying productivity in code bases that have different amounts of 'brittleness' (and code cycles). On some codebases developers even after understanding what is going on and understanding the change that needs to happen, developers can very easily spend hours making sure a line of code will actually work in different situations.