When I was studying in the high school, there was no argument about the source of the information we study, since it was very basic, we assumed it is 100 percent correct and it was kind of black and white. Later on when I joined the faculty of engineering, still we study black and white information with different methods of implementations, the grey area was very limited and this was suitable for BSc level. What I learned so far in my MSc study, there is nothing completely right or wrong; it depends on Expert judgment and how you are going to implement the information and the working environment.
There are three model types identified by (Wysocki 2012) which control the five Project Management Life Cycle. Only 20 percent of the projects falls under the Traditional Project Management (TPM) approach, where the Linear Project Management Life Cycle Model and Incremental Project Management Life Cycle Model applies. The other 80 percent of the projects falls under the Agile Project Management (APM) Approaches, where the Iterative Project Management Life Cycle Model and Adaptive Project Management Life Cycle Model applies. In addition to, Extreme Project Management Approach (xPM), where The Extreme Model applies.
The duration task estimation is a challenge target to the project team. It does not has neither fixed rules nor clear assumptions to follow, it is related to task resources allocation, and could vary from person to another and from project to another. (Wysocki 2012)