Despite what is often the case in software organisations, a Product Owner is not a Scrum Master. To me, the two roles are both absolutely critical to the success of a scrum team, but they are not only very different, but quite often contradictory.
The Scrum Master protects the team from interruptions and blockers and facilitates their working together both within the team and with other teams. The Product Owner sets the direction of the team and the vision of the product and defines and prioritises the backlog based on product needs and input from stakeholders.
Mike Cohn describes it by drawing a comparison to a pirate ship: if you consider the ‘management’ tasks that need to be taken care of on board Blackbeard’s Queen Anne’s Revenge, there were two main types: Star tasks (risk-taking, planning and strategic), and Guardian tasks (protective, ‘crew’ support tasks).
The team should see and know the difference…
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