Once you've got 7 people in a decision-making group, each additional member reduces decision effectiveness by 10%, according to Marcia W. Blenko, Michael C. Mankins, and Paul Rogers, authors of Decide & Deliver: 5 Steps to Breakthrough Performance in Your Organization. Thus, a group of 17 or more rarely makes any decisions.

Source: Decide & Deliver: 5 Steps to Breakthrough Performance in Your Organization