Reginald Braithwaite - Which theory fits the evidence?:

"Theory P [= probabilistic] adherents believe that the normal case for software projects is that tasks are rarely completed exactly as estimated, but that as a project progresses, the aggregate variance from estimates falls.

Theory D [= fully deterministic] adherents believe that the most important element of successful software development is planning. If a plan is properly constructed for the design and development of a software project, the actual implementation is virtually guaranteed."