From the Editor
IEEE Software, November/December 1999
Brooks' Law Repealed?
For more than 20 years, industry experts
have been reciting Brooks’ Law as gospel: Adding people to a late software
project is like pouring gasoline on a fire--it just makes it later. Twenty
years after the initial publication of The Mythical Man-Month, Fred
Brooks reiterated that Brooks’ Law was still "the best zeroth order
approximation to the truth."1
In spite of Brooks’ Law, adding people to a late project remains
commonplace. A study of software projects in the United Kingdom found that
about half of the managers of "runaway projects" (projects that exceeded
their planned schedules or budgets by more than 30 percent) attempted to
bring their projects under control by adding staff.2
When junior project managers try to rescue a late project by adding more
staff, wizened project managers and consultants intone the mantra: "Mustn’t
add people to a late project; adding people will make the project later." I
have evangelized this well-worn software engineering chestnut many times
myself, but I no longer think it’s true.
Brooks’ law is based on the idea that communications overhead is a
significant factor on software projects, and that work on a software project
is not easily partitioned into isolated, independent tasks. Ten people can
pick cotton ten times as fast as one person because the work is almost
perfectly partitionable, requiring little communication or coordination. But
nine women can’t have a baby any faster than one woman can because the work
is not partitionable. Brooks argues that work on a software project is more
like having a baby than picking cotton. When new staff are brought into a
late project, they aren’t immediately productive, and they must be trained.
The staff who must train them are already productive, but they lose
productivity while they’re training new staff. Brooks argues that, on
balance, more effort is lost to training and additional coordination and
communications overhead than is gained when the new staff eventually becomes
To those of us who have been around the software-project block a few times,
the claim feels true. We’ve participated on projects in which new people are
brought on at the end. We know the irritation of having to answer questions
from new staff when we’re already feeling overwhelmed about our own work.
We’ve seen new hires make mistakes that set the whole project back. And
we’ve experienced additional schedule slips even after staff has been added
to a late project. A typical scenario looks like this:Project start: A schedule of 12 months is established for the
Month 9: The project manager proposes adding two people, but the
developers say that they’re 75 percent done with the project and adding new
staff in the final stages of the project will make the project later.
Month 11: The project team acknowledges that it isn’t going to make its
12-month release date. After a heated debate about Brooks’ Law, the project
manager adds two people and reschedules the project for completion at the
Month 14: The project isn’t ready to be released. Developers chide the
project manager for violating Brooks’ Law.
Month 16: The project is finally released with significantly less
functionality than originally planned.
Did Brooks’ Law contribute to this
project’s schedule problems? Resolution of the issue turns on project
estimation, project tracking, and training.
The Standish Group’s Chaos report found that the average business systems
project overran its original schedule by 120 percent.3 Estimation accuracy on a typical software project
is terrible. And it’s actually worse than this statistic suggests because
more than half of the late projects trimmed significant functionality in
order to meet their 120-percent-behind-schedule result. Those projects would
have been even later if they had delivered all of their originally planned
and estimated functionality.
At Month 11, without effective project estimation in place, the plan to
release the software at the 14-month mark is an exercise in ungrounded
fantasy. When the software is finally released at Month 16, everyone knows
that the project was completed later than its revised target of 14 months,
but what no one can know with any certainty is what would have happened if
new hires hadn’t been added to the project. The claim is that adding staff
to a late project makes it later--but later than what? Later than a
systematic, well-founded estimate, or later than an estimate that was
optimistic by more than 100 percent in the first place?
The near-universality of poor project tracking (knowing a project’s status
with some degree of clarity) also undermines Brooks’ Law. If we begin a
12-month project with five people instead of three, we incur some per-person
reduction in productivity, but total productivity will be higher. What if we
add two people to a 12-month project at the end of Month 2? That’s still
early in the project and Brooks’ Law doesn’t apply. Organizations such as
NASA’s Software Engineering Laboratory recommend starting software projects
with a small senior staff and adding staff after initial requirements and
architecture work is mostly complete.4 Obviously it’s
beneficial to add staff until some point in the project’s schedule, after
which adding staff becomes detrimental. Implicit in Brooks’ Law 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?
We’ve all participated in projects in which we arrived at the planned
release date, took a three-week schedule slip, then continued slipping for
six months or more after that. At the time of the first schedule slip, we
don’t want to add new staff to the "late project" because three weeks
wouldn’t be enough time for them to become productive. But six months later
we wish that we had added the new staff, because six months would have been
more than enough time for them to become productive. At Month 9 in the
scenario above, without effective project tracking in place, the project
manager can’t know with any clarity whether the project is 75 percent
complete or 25 percent complete. Studies by the Software Engineering
Institute have found that poor project tracking is nearly universal in
"chaotic" projects (which are the vast majority).5 Most projects think they’re 90 percent complete
for the last 50 percent of the project, which obviously can’t be the case.
Because of widespread poor tracking, people think they’re at risk from
Brooks’ Law for significantly longer periods than they really are.
For Brooks’ Law to be true, the amount of training effort required from
existing staff must be significant. The amount of effort lost to training
must exceed the productivity contributed by new staff when they eventually
become productive. In Brooks’ example, three people work for two months,
then two more people are brought into the project, requiring one month of
training each. This is absurdly conservative. How could the first three
people possibly have done enough work in just two months to require a whole
month of training for new staff members? The time that existing staff spends
with new staff certainly does take productive time away from the project,
and time spent training can be frustrating to existing staff members who are
already feeling pressured to complete their own work. But the loss is
nothing like the ratio needed for Brooks’ Law to be true.
Zeroth-Order Approximation or Just Zero?
"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. The additional people will help, not hurt.
Controlled projects are less susceptible to Brooks’ Law than chaotic
projects. Their better tracking allows them to know when they can safely add
staff and when they can’t. Their better documentation and better designs
make tasks more partitionable and training less labor intensive. They can
add staff later in the project with less risk to the project.
Is Brooks’ Law the best zeroth-order approximation to the truth? I don’t
think so. Every project eventually reaches a point at which adding staff is
counterproductive, but that point occurs later than Brooks’ law states and
in limited circumstances that are easily identified and avoided.
1. F.P. Brooks, Jr., The Mythical
Man-Month, anniversary ed., Addison-Wesley, Reading, Mass., 1995.
2. A. Cole, "Runaway Projects¾ Cause and Effects," Software World,
Vol. 26, No. 3, pp. 3- 5, 1995.
3. The Standish Group, Charting the Seas of Information Technology,
The Standish Group, Dennis, Mass., 1994.
4. Recommended Approach to Software Development, Revision 3, Tech.
Report SEL-81-305, NASA Goddard Space Flight Center, Greenbelt, Md., 1992.
5. Software Eng. Inst., Process Maturity Profile of the Software
Community¾ 1998 Year End Update, Software Eng. Inst., Pittsburgh, Pa.,
Editor: Steve McConnell, Construx Software, 11820 Northup
Way #E200, Bellevue, WA 98005.