Brooks' law is a principle in software development which says that "adding manpower to a late software project makes it later". It was coined by Fred Brooks in his 1975 book The Mythical Man-Month. The corollary of Brooks's Law is that there is an incremental person who, when added to a project, makes it take more, not less time. Brooks adds that "Nine women can't make a baby in one month".
According to Brooks himself, the law is an "outrageous oversimplification", but it captures the general rule. Brooks points to two main factors that explain why it works this way:
- It takes some time for the people added to a project to become productive. Brooks calls this the "ramp up" time. Software projects are complex engineering endeavors, and new workers on the project must first become educated about the work that has preceded them; this education requires diverting resources already working on the project, temporarily diminishing their productivity while the new workers are not yet contributing meaningfully. Each new worker also needs to integrate with a team composed of multiple engineers who must educate the new worker in their area of expertise in the code base, day by day. In addition to reducing the contribution of experienced workers (because of the need to train), new workers may even have negative contributions – for example, if they introduce bugs that move the project further from completion.
- Communication overheads increase as the number of people increases. The number of different communication channels increases rapidly with the number of people. Everyone working on the same task needs to keep in sync, so as more people are added they spend more time trying to find out what everyone else is doing.
Exceptions and possible solutions
Brooks' law is often cited to justify why projects keep being late, despite management efforts. However, there are some key points in Brooks' law that allow exceptions and open the door for possible solutions.
The first point is to note that Brooks's law often applies to projects that are already late. Projects can be brought back into (or kept in) control if people are added earlier in the process. It is also important to determine if the project is really late, or if the schedule was originally overly optimistic. Scheduling mistakes account for a large number of late projects. Correcting the schedule is the best way to have a meaningful and reliable time frame for the project's completion.
The quantity, quality and role of the people added to the project also must be taken into consideration. One simple way to circumvent the law on an overrun project is to add more people than needed, in such a way that the extra capacity compensates the training and communication overhead. Good programmers or specialists can be added with less overhead for training. People can be added to do other tasks related with the project, for example, quality assurance or documentation; given that the task is clear, ramp up time is minimized.
Good management and development practices also help to minimize the impact of Brooks' law. The modern practices of continuous integration, test first design, and iterative development significantly reduce the inter-developer communication overhead, and thus allow for better scalability. New tools for software development and documentation also help to minimize the ramp up time, making it simpler for new programmers to get involved in the work. Design patterns simplify the distribution of work, because the entire team can do its part within the framework provided by that pattern. The design pattern defines the rules that the programmers follow, simplifies communication through the use of a standard language, and provides consistency and scalability. Finally, good segmentation helps by minimizing the communication overhead between team members. Smaller sub-problems are solved by a smaller team, and a top-level team is responsible for systems integration. For this method to work, the segmentation of the problem must be done correctly in the first place; if done incorrectly, this can make the problem worse, not better, by impeding communication between programmers working on parts of the problem which are actually closely coupled, even when the project plan has decreed that they are not.
Some authors – see, for example, Creating a Software Engineering Culture by Karl E. Wiegers – have stressed the importance of the social and political aspects of the work climate as determinants of the effectiveness of individual programmers and the project team as a whole. Rather than depending on heroes to carry the day with extraordinary efforts, Wiegers argues that a team of ordinarily-skilled individuals can repeatedly deliver timely results in the right work environment. Efforts to improve the effectiveness of teams can ameliorate, if not eliminate, the consequences of Brooks's law.
Open source software development
While open source projects rarely have schedules, nonetheless they can reach a state in which they are called "late" by their sponsors, participants, and users. In such a case, Brooks' law ("adding manpower to a late software project makes it later") surely applies, for exactly the reasons that Brooks enumerates: time for the new developers to become productive and increased communication overhead. In addition, unless there are strict controls, newcomers may reduce the productivity of experienced developers by checking in buggy or inappropriate changes, which then have to be backed out.
- Frederick P. Brooks, Jr. "The Mythical Man-Month". 1995 . Addison-Wesley.
- James Taylor, "A Survival Guide for Project Managers", 2nd edition, AMACOM, 2006, ISBN 978-0814408773, p.21. The number of communication channels increases with the number of people, n, according to the formula: number of communication channels = n(n-1)/2
- "In spite of Brooks' law, adding people to a late project remains commonplace" ... "I have evangelized this well-worn software engineering chestnut many times myself, but I no longer think it's true". (McConnell, 1999)
- "The trouble is that there are important exceptions that many people do not take the time to consider when using Brooks's law to justify something". (Berkun, 2006)
- "Implicit in those projects is that it applies only to the final phases of a project. The question is, How do you know whether you're in a project's final phases?" (McConnell, 1999)
- "We have found that adding people to a late project will always increase its cost, but the project may not always be late since there may be sufficient schedule to absorb them and the project may not be at maximum staffing. Only under certain degree of sequential constraints among project tasks will the project be delayed." (Hsia, Hsu, Kung, 1999)
- Late chaotic projects are likely to be much later than the project manager thinks – project completion isn't three weeks away, it's six months away. Go ahead and add staff. You'll have time for them to become productive. Your project will still be later than your plan, but that's not a result of Brooks' law. It's a result of underestimating the project in the first place." (McConnell, 1999)
- "Gordon and Lamb studied Brooks's law and suggested that the best way to recover from a slipping schedule is to add more people than might be expected to be necessary, and to add them early." (Hsia, Hsu, Kung, 1999)
- "The law assumes that all added manpower is equal, which is not true. Given the choice of adding a good programmer, who knows the code base and is friends with half the team, I'd consider it." (Berkun, 2006)
- "The sad but popular approach is to throw people in without much explanation and let everyone figure it out for themselves. But if the manager clarifies why Sally and Rupert are joining, and defines good roles for them, with input from the team, they'll be set up to make a smooth transition." (Berkun, 2006)
- Steve McConnell. "Brooks' law Repealed," IEEE Software, vol. 16, no. 6, pp. 6–8, Nov/Dec, 1999. Also available at the authors website (Brooks's law repealed?).
- Pei Hsia, Chih-tung Hsu, David C. Kung. "Brooks's law Revisited: A System Dynamics Approach," compsac, p. 370, Twenty-Third Annual International Computer Software and Applications Conference, 1999.
- R. L. Gordon and J. C. Lamb. "A Close Look at Brooks' law," Datamation, June 977, pp. 81–86.
- Berkun, Scott (January 11, 2006). "Exceptions to Brooks' Law". Retrieved 2008-07-28.
- Brooks law Is Applicable To Many Collaborative People Activities