How do you define your key team members?

Key team members are those who are "KEY" to get the job done.  This implies a comparison and math/mismatch between the "credentials" of the members and the job requirements in terms of team member capabilities and skills. It would also help if the "job requirements" are also categorized according to the mission/vision analysis into "critical", "good to have", or "nice to have" If you need to know more about it, let me know !