What is the optimal size of a team?
The Scrum Guide currently sets the size of the Development Team from three to nine members, around the number six. Others say seven is the magic team size. What is the rationale for such a number and where does it come from?
Optimal Team Size
In software development there’s a term called “Brooks’s Law” that Fred Brooks first coined back in 1975 in his seminal book The Mythical Man-Month. Put simply, Brooks’s Law says “adding manpower to a late software project makes it later.” This has been borne out in study after study. Lawrence Putnam is a legendary figure in software development, and he has made it his life’s work to study how long things take to make and why. His work kept showing that projects with twenty or more people on them used more effort than those with five or fewer. Not just a little bit, but a lot. A large team would take about five times the number of hours that a small team would. He saw this again and again, and in the mid-1990s he decided to try to do a broad- based study to determine what the right team size is. So he looked at 491 medium-size projects at hundreds of different companies. These were all projects that required new products or features to be created, not a repurposing of old versions. He divided the projects by team size and noticed something right away. Once the teams grew larger than eight, they took dramatically longer to get things done. Groups made up of three to seven people required about 25 percent of the effort of groups of nine to twenty to get the same amount of work done. This result recurred over hundreds and hundreds of projects. That very large groups do less seems to be an ironclad rule of human nature.
Scrum : The Art of Doing Twice the Work in Half the Time, page 46. 2014.
Scrum@Scale
The Scrum@Scale guide refers to another study to set the optimal number of teams in a Scrum of Scrums (4 or 5):
Harvard research has determined that optimal team size is 4.6 people (on average) [...].
J. Richard Hackman. Leading Teams: Setting the Stage for Great Performances. Harvard Business Review Press. 2002.
The Magical Number Seven
We often refer to the magical number seven. As far as I know, it originates from an old paper from George Miller (Harvard University), read as an Invited Address before the Eastern Psychological Association inPhiladelphia on April 15, 1955. The exact title is:
The Magical Number Seven, Plus or Minus Two, Some Limits on Our Capacity for Processing Information
It’s summary ends with this funny remark:
And finally, what about the magical number seven? What about the seven wonders of the world, the seven seas, the seven deadly sins, the seven daughters of Atlas in the Pleiades, the seven ages of man, the seven levels of hell, the seven primary colors, the seven notes of the musical scale, and the seven days of the week? What about the seven-point rating scale, the seven categories for absolute judgment, the seven objects in the span of attention, and the seven digits in the span of immediate memory? For the present I propose to withhold judgment. Perhaps there is something deep and profound behind all these sevens, something just calling out for us to discover it. But I suspect that it is only a pernicious, Pythagorean coincidence.
Dunbar number
Finally, when describing their design of scaling Agile at Spotify in 2012, Henrik Kniberg and Anders Ivarsson introduced another number:
Tribes are sized based on the concept of the “Dunbar number”,which says that most people cannot maintain a social relationship with more than 100 people or so (the number is actually larger for groups that are under intense survival pressure, which isn’t really the case at Spotify, believe it or not...). When groups get too big, we start seeing more things like restrictive rules, bureaucracy, politics, extra layers of management, and other waste.
Scaling Agile @ Spotify with Tribes, Squads, Chapters & Guilds. Henrik Kniberg and Anders Ivarsson. October 2012.