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Succeeding with Agile Software Development Using Scrum: Team Structure

In this chapter Mike Cohn looks at the importance of two critical factors to be considered when deciding how to structure Scrum teams: keeping teams small and orienting each team around the delivery of end-to-end user-visible functionality
This chapter is from the book

It is perhaps a myth, but an enduring one, that people and their pets resemble one another. The same has been said of products and the teams that build them.

  • The system being produced will tend to have a structure that mirrors the structure of the group that is producing it, whether or not this was intended. One should take advantage of this fact and then deliberately design the group structure so as to achieve the desired system structure. (Conway 1968; commonly referred to as "Conway's Law")

If it is true that a product reflects the structure of the team that built it, then an important decision for any Scrum project is how to organize those individuals into teams. Factoring into this decision are considerations of team size, familiarity with the domain, the channels of communication, the technical design of the system, individual experience levels, the technologies involved, the newness of those technologies, where team members are located, competitive and market pressures, expectations about project schedule, and much more.

In this chapter we look at the importance of two critical factors to be considered when deciding how to structure Scrum teams: keeping teams small and orienting each team around the delivery of end-to-end user-visible functionality. We also look at the importance of having the right people on each team and not overloading those individuals by forcing them to split time among too many teams. We conclude the chapter with nine questions to ask when starting a multiteam project.

Feed Them Two Pizzas

I was working on a project for a bioinformatics company when the CEO asked me to provide her with an estimate of how long the project would take. The application was large, the domain complicated, and the team mostly new. Because the domain was so complicated, our team was made up of some very smart Ph.D. scientists, who knew only a little about programming, and some very smart programmers, most of whom had taken no more than a class or two in biology or genetics. No one on the team was great at both the science and the development.

After a bit of research and work with the team I returned to the CEO with an estimate of something like 100 person-years. In other words, if we used all 40 people on the team, we could finish the project in about two and a half years. I don't think that number was too shocking to her, but it was a big number, so she asked me, "What's the cheapest way we could write it?" My answer: "Take Steve, the scientist with the best understanding and aptitude for programming, and have him go spend 10 years working in a great software company doing nothing but learning how to be a great programmer. Then have him return to our company and spend 30 years working alone to write the program. It'll take 40 years, but it's your cheapest option." She should have been quite pleased with my answer—after all, I'd taken the 100 person-year initial estimate and offered her a way to cut it by more than half. Alas, 40 years was just a bit too long for her to wait.

As this story illustrates, a team offers the advantage of getting things done far more quickly than one person could, but with that advantage comes a potentially large amount of communication overhead. Knowing that, what is the ideal team size for Scrum projects? Generally accepted advice is that the ideal Scrum team size is five to nine individuals. While I agree with this, putting a number to it makes me nervous. If you're thinking about your ten-person team right now you may feel inclined to return this book, demand a refund, and give up on Scrum.


Rather than take the five-to-nine person guideline too literally, I prefer how Amazon.com thinks its about its teams. Amazon refers to them as "two-pizza teams," meaning a team that can be fed with two pizzas (Deutschman 2007). As humorous as that is, it's actually useful. If ordering food for the occasional team lunch is a hassle, it could be a good indicator that the team has become too large.

The largest single Scrum team that I worked with where I was content to leave them alone was 14 people. The team, its ScrumMaster, and I had all looked at possible ways to split them up, but no solutions we came up with seemed better than leaving them intact. I've also worked with one team of 25 that insisted it should be one team rather than more. They were wrong; there was too much communication overhead on a single team of that size.

Why Two Pizzas Are Enough

To be fair, there are some advantages to large teams. Large teams may include members with more diverse skills, experiences, and approaches. Large teams are not as much at risk to the loss of a key person. They may also provide more opportunities for individuals to specialize in a technology or a subset of the application.

On the other hand, there are even more advantages to small teams. These include the following:

  • There is less social loafing. Social loafing is the tendency for people to exert less effort when they believe there are others who will pick up the slack. Members of small teams are less prone to social loafing. Social loafing was first demonstrated by psychologist Max Ringelmann in the 1920s when he measured the pressure exerted by individuals and teams pulling on a rope. Groups of three exerted only two-and-a-half times (not three times) the average individual pressure. Groups of eight exhibited less than four times the individual average. Ringelmann's and related studies have shown that individual effort is inversely related to team size (Stangor 2004, 220).
  • Constructive interaction is more likely to occur on a small team. Stephen Robbins, author of Essentials of Organizational Behavior, a best-selling textbook on organizational behavior, has concluded that teams of more than 10 to 12 people have a difficult time establishing feelings of trust, mutual accountability, and cohesiveness. Without these, constructive interaction is difficult (2005).
  • Less time is spent coordinating effort. Small teams spend less time coordinating the efforts of team members. This is true both in the aggregate and as a percentage of total project time. As a simple example, we all know that the effort just to plan a meeting for a large team can be overwhelming.
  • No one can fade into the background. With large teams, there is lower participation in group activities and discussions. Similarly, the disparity in the amount of participation among team members increases. The problems can prevent a group of individuals from jelling into a cohesive, high-performing team.
  • Small teams are more satisfying to their members. With a small team, one person's contributions are more visible and meaningful. This is perhaps one reason why research has shown that participation on a large team is less satisfying to team members (Steiner 1972).
  • Harmful over-specialization is less likely to occur. On a large project, individuals are more likely to take on distinct roles (Shaw 1960). For example, one developer chooses to work only on the user interface. This creates wasteful hand-offs of work between team members and reduces the amount of learning that occurs when individuals are more willing and likely to work beyond specific job roles.

One interesting study of team size looked at 109 different teams. The small teams had 4 to 9 members while the large teams had 14 to 18. The researchers reached several conclusions.

  • Members of smaller teams participated more actively on their team; were more committed to their team; were more aware of the goals of the team; were better acquainted with other team members' personalities, work roles, and communication styles; and reported higher levels of rapport. The data also show that larger teams are more conscientious in preparing meeting agendas compared to smaller teams. (Bradner, Mark, and Hertel 2003, 7)

Hmm. With a small team I can have many compelling advantages. Or I can staff a larger team and get better meeting agendas.

Small Team Productivity

Given the strength of these advantages to small teams, we would expect small teams to be more productive than large teams. Doug Putnam of QSM found exactly that after studying 491 projects with team sizes from 1 to 20 people. Since 1978 QSM has been collecting data on software productivity and estimates. The company maintains the software development industry's most thorough metrics database, including data on application size, effort, industry, and more. As such, the QSM database is uniquely valuable for comparing different types of projects.

From the QSM database of over 7,000 projects, Putnam narrowed the data set to 491 projects completed between 2003–2005 that delivered between 35,000 and 95,000 new or modified lines of source code.1 Project sizes were evenly distributed from 1 to 20 team members. As shown in Figure 10.1, Putnam found that the smaller the team size, the more productive each team member was. However, the difference between teams sized from 1.5 to 7 people was very small.

Figure 10.1

Figure 10.1 The average productivity per person on teams of various sizes. Printed with permission from QSM, Inc. All rights reserved.

Putnam looked also at the total development effort that goes into projects. Not surprisingly, he found that smaller teams complete projects with less total effort. Putnam concluded that "larger teams translate into more effort and cost. The trend appears to have an exponential behavior. The most cost-effective strategy is the smallest team; however the extreme nonlinear effort increase doesn't seem to kick in until the team size approaches nine or more people." These results can be seen in Figure 10.2.

Figure 10.2

Figure 10.2 Smaller teams require less total effort to deliver the same size project. Printed with permission from QSM, Inc. All rights reserved.

In most cases, however, we are not concerned with minimizing the total development effort; schedule is always a major consideration. After all, we rarely have 40 years to wait for a lone developer to finish what we need by next spring. The impact of team size on overall schedule is shown in Figure 10.3. This figure shows that a 5- to 7-person team will complete an equivalently sized project in the shortest amount of time. Smaller teams took slightly longer. Notice again the dramatic increase with teams of 9 to 11 people.

Figure 10.3

Figure 10.3 Teams of five to seven people finished equivalently sized projects in the shortest amount of time. Printed with permission from QSM, Inc. All rights reserved.

An additional study described in the Communications of the ACM compared the productivity of large and small teams. Long-time industry veteran Phillip Armour writes of this research.

  • Large teams (twenty-nine people) create around six times as many defects as small teams (three people) and obviously burn through a lot more money. Yet, the large team appears to produce about the same amount of output in only an average of twelve days less time. This is a truly astonishing finding, though it fits with my personal experience on projects over thirty-five years. (2006, 16)

With all of the strong reasons in favor of small teams, I don't think I'll be placing any orders for three pizzas any time soon.

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