What is the optimal group size for decision-​​making?

by Sheila Margolis on January 24, 2011

Much of work today is done in teams. Even in MBA programs, the team is the structure used to meet class goals. So what is the optimal size for effective decision-​​making? It appears that a recent Harvard Business Review stat published that research by Marcia W. Blenko, Michael C. Mankins, and Paul Rogers indicates that seven (7) is the optimal size. Yet much of the research I’ve found says that number is a bit too high.

First, many studies target an “odd” number as the first criteria for group size. According to one resource:

This (an odd number) prevents ties and improves the odds of making a correct decision when using majority rules.

Even-​​numbered groups can make decisions, but the decision-​​making can take more time.

Getting back to the actual number, think about the benefits of a large group. The more people you have, theoretically, the better chance you have of getting the best information to make the best decision. Research has shown that collective intelligence does exist. But, according to research reported in Science, the October 2010 issue  by authors Anita Williams Woolley, Christopher F. Chabris, Alex Pentland, Nada Hashmi and Thomas W. Malone:

This “c factor” (the group’s collective intelligence) is not strongly correlated with the average or maximum individual intelligence of group members but is correlated with the average social sensitivity of group members, the equality in distribution of conversational turn-​​taking, and the proportion of females in the group.

So it looks like social sensitivity–possibly a more common attribute to females–facilitates group decision-​​making. Emotional intelligence and what some consider the soft stuff is important to the functioning of teams. So getting back to the optimal group size–what’s the best number? In what size, can you have the equality in distribution of conversation turn-​​taking as the research indicates is an important feature of an effective group?

If you measure the number of possible social interactions with varying group sizes, the optimal group size appears to be five (5). According to a resource on applications of probability and statistics:

As can be seen by the figure below, the number of possible social interactions begins to explode in groups with more than 5 people.

Research by Hackman and Vidmar (1970) on optimum group size for member satisfaction showed a similar outcome. They composed groups that ranged in size from 2–7 members to assess the impact of size on group process and performance for various kinds of tasks. After the groups had finished their work, they asked participants independently to indicate the extent of their agreement with the following two questions: Question #1– This group was too small for best results on the task it was trying to do. Question #2– This group was too large for best results on the task it was trying to do. The chart below indicates the average answers to these two questions on the same graph. Not surprisingly, few people in the dyad thought it was too large and few in the 7-​​person group thought it was too small. What is noteworthy is where the two lines crossed. They dropped a perpendicular line from that point to the horizontal axis and discovered that the optimum group size was 4.6 members.

So if you’re looking for the best size for a team, consider an odd number close to five. But remember the number is just one factor. Social sensitivity and being able to read emotions are attributes of successful team decision making. Consider the number and consider the members. Maybe they’ll need a little training in empathy and being sensitive to others as well as having a culture that allows all to fully participate. Sounds like the right-​​sized team that practices many of the principles of employee engagement can be the most effective.

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