PPT Slide
The second problem with estimating completion time of software projects is with the unit of time used to estimate the completion of the project (i.e., MAN-MONTH).
There is a fallacy involved with this unit: It is true that the cost of the project will increase proportional to the MAN-MONTH used for that project. However, the progress is not going to be achieved proportional to the number of MAN-MONTHS used for a project.
Therefore, it is a false assumption that by adding more MAN-MONTHS you can complete the project earlier.
MAN and MONTH are in fact interchangeable, only if the project consists of several independent efforts (INDEPENDENT PARTITIONED TASKS) and a case where team members do not have to communicate with each other.
e.g.: Men for Months Man-Month
In fact, one of the problems with software engineering projects is that as we add more MAN-MONTHS, the amount of communication among the men is going to increase rapidly.