A Product Owner arrives at refinement with sizes already written against the top ten Product Backlog items, taken from a similar product they ran last year. The Developers think several are badly wrong. Who is responsible for sizing in Scrum?

Professional Scrum Master I, objective 1. Understanding and applying the Scrum framework medium

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The options

Correct The Developers who will do the work — the Product Owner may influence them by helping them understand and select trade-offs

Correct. The people who will do the work are the only ones who can judge what it will take, and Scrum names them explicitly so that the estimate cannot be handed down.

Not correct The Product Owner, because size feeds the ordering decision that they own

Wrong. Ordering is the Product Owner's and sizing is not, and the separation is deliberate: value and cost are judged by different people, which is what keeps the conversation about trade-offs honest.

Not correct The Scrum Master, as a neutral party with no stake in the outcome

Wrong. A Scrum Master helps the team find effective techniques for Product Backlog management; supplying the numbers would make them a participant in exactly the decision they are meant to help the team own.

Not correct Whoever in the organisation has the most experience with this kind of work, regardless of their role

Wrong. Outside experience is useful input and the Developers are free to seek it, but the accountability stays with the people who will actually build the thing in this team, with this codebase.

Why

Refinement is where this plays out: breaking items down and adding detail such as a description, order and size. Order comes from the Product Owner, size comes from the Developers, and the Product Owner's lever on size is persuasion about trade-offs — make it smaller, drop this part, accept that constraint. A team where one person supplies both numbers has lost the tension the two accountabilities exist to create.

Where this comes from

Cited
Scrum Guide section Product Backlog
What it says
The Developers who will be doing the work are responsible for the sizing.

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