One school is that everything is fully deterministic in practice (“Theory D”). If development appears, from the outside, to be probabilistic, it is only because we haven’t discovered the “hidden variables” that fully determine the outcome of software development projects. And, since we are talking about development in practice, it is practical to measure the variables that determine the outcome such that we can predict that outcome in advance.

The other school of thought is that development is fully probabilistic in practice (“Theory P”), that there are no hidden variables that could be used to predict with certainty the outcome of a software development project. Theory P states that the time and effort required to measure all of the variables influencing a software development project precisely enough to predict the outcome with certainty and in advance exceeds the time and effort required develop the software.

…

To date, Theory P is the clear winner on the evidence, and it’s not even close. Like any reasonable theory, it explains what we have observed to date and makes predictions that are tested empirically every day.

Theory D, on the other hand, is the overwhelming winner in the marketplace, and again it’s not even close. The vast majority of software development projects are managed according to Theory D, with large, heavyweight investments in design and planning in advance, very little tolerance for deviation from the plan, and a belief that good planning can make up for poor execution by contributors.

Does Theory D reflect reality? From the perspective of effective software development, I do not believe so. However, from the perspective of organizational culture, theory D is reality, and you ignore it at your peril.

– http://weblog.raganwald.com/2007/06/which-theory-first-evidence.html