The fundamental unit of Scrum is a small team of people, a Scrum Team. The Scrum Team consists of one Scrum Master, one Product Owner, and Developers. Within a Scrum Team, there are no sub-teams or hierarchies. It is a cohesive unit of professionals focused on one objective at a time, the Product Goal.
The entire Scrum Team is accountable for creating a valuable, useful Increment every Sprint. Scrum defines three specific accountabilities within the Scrum Team: the Developers, the Product Owner, and the Scrum Master.
The roles all equally important within the Scrum framework, each with their own focus, responsibility and services to the other roles. Together, they form a cross functional and self managing team that can create great products and services within complex environments.
Leaving out one of the roles means there is no Scrum Team, therefore no Scrum
Combining the roles makes the team less effective and even disable it, despite of some possible efficiency gains
Each role serves the two other roles in different ways making a combined effective Scrum Team.
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