The ideal time for refinement to take place and who should participate are the Scrum Team during the current Sprint, if they have been unable during preceding Sprints to define Product Backlog items with enough precision to begin work, and the Scrum Team on an ongoing basis, defining Product Backlog items into smaller more precise items that are ready for selection. This is because:

Product Backlog refinement is the act of adding detail, estimates, and order to Product Backlog items. It is an ongoing and collaborative process that involves the whole Scrum Team.

The Scrum Team should refine the Product Backlog as needed to ensure that there are enough items that are sufficiently clear and ready for selection in Sprint Planning. The amount of time spent on refinement usually varies from one Scrum Team to another, but it is recommended not to exceed 10% of the capacity of the Developers.

The Scrum Team may also use a Sprint to refine the Product Backlog if they have not been able to do so in previous Sprints. This may happen when the product is new or complex, or when there is a significant change in the product vision or direction. However, this should be an exception rather than a rule, as it may delay the delivery of value and feedback.

Other options, such as business analysts doing the refinement work for the Scrum Team, the Product Owner doing the refinement alone in Sprint 0 or between Sprints, or handing off the ownership of the Product Backlog to someone else, are not consistent with the principles and values of Scrum. They may lead to misunderstandings, misalignment, or loss of ownership and accountability.

References:

[Scrum Guide], page 11, section “Product Backlog”

[Scrum Guide], page 12, section “Product Backlog Refinement”

[Professional Scrum Product Owner™ Training], page 7, section “Product Backlog Management”

[Professional Scrum Product Owner™ Training], page 8, section “Release Management”

[Professional Scrum Product Owner™ Training], page 9, section “Business Strategy”