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Ep23. What is the ideal agile team size?

I want to share about the ideal team size, or Dunbar’s number.

It’s the limit on the number of people with whom one can form effective, stable Social Relationships; first published in 1992 by anthropologist Robin Dunbar.

Dunbar noted a correlation between the size of a primate brain and the average social group size. By using the average human brain size and extrapolating from the results of primates, he estimated that humans can comfortably maintain ~150 stable relationships.

The term Social Relationship relates to regular interactions, the ability to recognize another member as part of the group, or are committed to a common goal.

These attributes are very important in Agile projects because they rely on team cohesion to minimize control mechanisms.

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Dunbar’s argues that at each level of closeness, we are limited in how many relationships we can have: an average of 5 intimate supportive relationships, 15 close friends and so on. We devote about two-thirds of our time to just 15 people.

Dunbar's number provides a limit to try to avoid letting work become too large to manage.

Based on this, 5 to 9 people is often referred to as the ideal team size, and BU’s should not exceed 150.

Some interesting stats from other research which seems to confirm the 150 number:

  • A study of the twenty tribal societies with available data showed a mean clan group size of 153

  • Surprisingly the average number of Facebook friends is 150–200

  • A 2011 twitter study of 1.7m users found they, maintain a stable relationship with 100–200 individuals

  • Exchange of Christmas cards in the UK & the maximum network size was about 153.5

  • 2008 survey by The Knot Wedding Network of over 18,000 brides revealed an average wedding guest total of 148

  • The roman army during the Republic utilized a fighting unit called the maniple with 130–140 solders & officers

  • Modern military companies tops at about 150

  • Middle Eastern Neolithic villages dating back to 6000BC usually populated 120–150

  • in the 1800s an average English village had about 160 residents

Dunbar’s number is somewhat controversial

Some scientists have questioned the relevance of brain size; there are other factors that play a role (e.g. territory size, diet). In fact, they’ve found a bewildering array of correlations between brain size and behavioural traits.

If you are keen, here is an entertaining video of Dunbar discussing his work.

This article was first published here.


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