What is the Recommended Scrum Team Size?

Nearly every client I work with asks me this question at some point. The Scrum Guide offers very limited guidance, suggesting 3-9 people per team (exclusive of ScrumMaster and Product Owner), without giving reasons or context for those numbers. There isn’t one, universally correct answer for optimal team size, but there are a number of factors and tradeoffs worth considering when figuring out what will work best for your team. In this post, we’ll explore the research and I’ll share my personal experiences about effective team size. While this is primarily about Scrum, the lessons are applicable for any work that is collaborative and knowledge-based. Effective Team Size is More than Just a Number When you’re trying to determine what will work best, your  Team’s needs should be more important than any arbitrary number. Factors you should consider include: A sufficiently broad set of skills to build their product – aka Cross-Functional Team members dedicated to one, and only one, team Stable membership – i.e. team membership that is consistent over a long period of time[1] Diversity of thought – a sufficiently broad set of beliefs, attitudes and thinking patterns[2] Once the Team is formed, these are often just as important as team size when predicting success: Psychological safety – the environment is safe for all team members to share their ideas[3] Collective Intelligence of the Team – strongly correlated with average sensitivity of team members[4] Equal Communication – the most expressive team member is no more than twice as expressive as the quietest[5] Open-Mindedness and Willingness to Learn[6] A Shared Vision of which all Team members are committed to achieving[7] Clear Roles and Responsibilities[8] Dynamically Shared Leadership[9] External Coach – in Scrum this is the ScrumMaster[10] These factors don’t exist in a void, so let’s explore the evidence that supports what others, including myself, have discovered about team size, to help you consider what’s most important for your Team. After all, there is no universal answer, there is only a range that will likely work best for your needs. The Problem with Arbitrary Numbers The earliest Scrum and XP books all suggest a team size number of 7+/-2, applying Miller’s number,[11] believed to be the number of integers you can hold in short-term memory. I’m troubled by this, since I can’t see why one’s ability to keep track of numbers should be correlated with team size. In addition, more recent research[12] has shown that, as the things you’re keeping track of become more complex, the number of items you can keep in your short-term memory drops to 4 or less. Clearly, we need more useful sources. Many other coaches cite historical examples going back to the Roman army, which used small military unit sizes of around 8 people. Others observe bonobos, one of the closest genetic relatives to humans, often split into groups of 6-7 to forage for a day. These both conveniently support the 7+/-2 number, but since neither example is about teams doing knowledge work, the relevance of these for Scrum is limited. What Happens to Relationships When Teams Get Large? In a team, each individual will connect with another individual and form a unique relationship. The bigger the team, the more the relationships. The equation that describes this is N (N – 1)/2. But what exactly does that mean? More importantly, how is it supposed to help you? Let’s start by treating it as a high-school math problem, then turn it into something we can actually use in real life. The above equation tells us how many different relationships will exist within a team of a certain size. N = the number of people in the team. So, in the first example of a team of 5 people, for N=5 you have 10 relationships: 10 different combinations of team members relating with other team members. In the second example, n=7, you have 21 relationships, and at n=9 in the third image you have 36 relationships.  Each pair of people represents one relationship and that relationship is how they collaborate. High-performing teams, by their nature, must have strong relationships between each of the team members to collaborative effectively. Why does this matter and how can it help you? Each new person adds some individual productivity to the Team, but they also increase the communications overhead in the form of an exponentially growing number of relationships. To increase a team from 5 to 7 people, you have to more than double the number of relationships. To go from 7 to 9, you don’t quite double it, but the jump is still large. Just how expensive is it to maintain these relationships? Anecdotally, having studied team member interactions at clients’ sites, I can say that in teams of 7-8 people, upwards of 90 minutes per person each day is spent interacting with other team members.[13] This excludes pair programming time. Some of the interaction is talking about work, but just as much is spent socializing. This is good and important, because it’s the combination of work and socializing that builds a team’s resilience and ability to handle challenges effectively (see the water cooler section in “Five Steps Towards Creating High-Performance Teams“). So the number of relationships between team members, and the time investment they require, needs to be a factor considered when choosing team size because it will influence productivity. A general rule of thumb suggests that people typically have from 3½ to 5 hours of productive time at work each day. As a team gets larger, we either lose productivity or, more often, we start to withdraw socially rather than sacrifice productive time for interacting with our peers, because the communication costs for each team member is becoming too high to afford. We need strong relationships to become a high-performing team but, as group size grows, we start to avoid the interactions that build those relationships.[14] Many Hands Make Light Work… But Only to a Point We all want to believe that if we go from 1 person […]