Man Months is one of the most misunderstood words in software industry. It’s used to refer to the effort required to execute a software project. Simply speaking, a man month refers to the effort of one developer working for a month. However, the concept goes beyond this simplicity.
Frederick Brooks’s The Mythical Man-Month: Essays on Software Engineering, published in 1975, is one of the most influential books on software engineering. Amazon website says
“Few books on software project management have been as influential and timeless as The Mythical Man-Month. With a blend of software engineering facts and thought-provoking opinions, Fred Brooks offers insight for anyone managing complex projects.”
This book has made the term “Mythical man months” extremely famous term though relevant. The basic premise of the book, also referred to as Brooks’ law:
“Adding manpower to a late software project makes it later.”
Which essentially means that man months is not a mathematical concept. So, if we estimated that a project is going to take 35 man months to complete, does not mean that putting 35 people on the project, will finish the project in 1 month.
Extending this concept further, if a project is getting executed, which was supposed to be 45 man months of effort, started with 5 members. After 3 months of work, we can say that 15 man months of effort is spent and the remaining effort is 30 man months. But this project had a deadline of 5 months. Now how do you finish the project in 2 more months (remember, 30 man months of effort is still pending)?
One may say, put 10 more people, making the team size to be 15. So 15 people working for 2 months will be equivalent to 30 months, done deal. Not True, this is what Brooks’ has discussed in great details in his book.
Sue Kim, in the blog post Mythical Man Month – The Cliff Notes mentions that:
“Men and months are interchangeable commodities only when a task can be partitioned among many workers with no communication among them.”
This points to an important aspect of human communication in a team. More people open up more channels of communication leading to errors and delays. Any business communication text will explain that 15 members in a team, will have 15 *(15-1)/2 = 135 possible ways of conversations amongst them, leading to confusion and errors.
In the blog post titled Modeling the Mythical Man-Month, a mathematical model has been mentioned to explain Books’ law. Another mathematical model namely Putnam Model was published immediately after Books’ book to provide a mathematical model.