The official scrum guide mentions about carrying out routine maintenance activities to update the product backlog, or to carry out the product backlog refinement. The exact time to be invested in the grooming activity depends upon the management, and how scrum is to be implemented in the project. A rule-of-the-thumb followed is to put in approximately 10% of the time utilized during the sprint activity, into the grooming activity. It is important to be clear regarding some of the aspects associated with product backlog refinement.

Purpose and goals of carrying out the refinement
The primary reason why the product backlog should be refined is to update or rebuild the backlog so that it remains consistent with the requirements provided by the stakeholders with regards the new features and functionality to be included in the project. Another reason is to review existing user stories or product backlog items and decide whether they are still useful or pertinent from the development point of view, and to update the acceptance criterion and the explanation detailed in each PBI.

It is recommended to use the “DEEP” method – detailed appropriately, estimated, emergent, and properly ordered – while prioritizing the user stories within the backlog. Larger stories or epics should be systematically broken down in to more manageable smaller ones, proper estimation by assigning relevant story points to the PBIs should be carried out, user stories should be rearranged as per the new priorities, and the queries regarding the development of user stories during the sprint should be effectively answered by the product owner. Whenever a meeting is planned to refine the PBIs, the objective should be to carry out enough refinement work so that it lasts for at least three future sprints.

Duration and frequency of the grooming activity
Each activity and meeting is time boxed in scrum. Following the same principle, the product backlog refining or grooming activity should be time boxed too. However, in practice, there is no pre-designated activity or a meeting for planning and carrying out the product backlog refinement activity in the same manner as the sprint planning meeting and the sprint retrospective meeting is held. Backlog grooming is carried out more as a routine activity than anything else in scrum, and the guide does not exactly specify how much time or efforts should be invested in the activity. Perhaps a possible reason could be that the product development and creation of product backlogs vary from project to project, and it is difficult to standardize how the grooming activity should be carried out since the size and nature of the product backlog cannot be adjudged.

In practice, ideally time equivalent to 10% of the total time spent during the sprint activity should be allotted for the product refinement. For a two week sprint consisting of a total of 6 working hours per day and 14 sprint days per sprint, the time to be allotted should be approximately 10% of 6 hours x 14 days = 8.4 hours (10% of 6 hours x 14 days = 84 hours). This could be rounded up to one working day. Since the refinement activity is to be carried out on a consistent basis, investing additional time could lead to decreased productivity and an extended product release date – something that should be avoided. In actual practice, this rule suffices to a great extent.

Who should participate in the grooming activity?
Besides the product owner, the grooming sessions should be attended by the development team members and the scrum master. The stakeholders can participate in the sessions too, but their participation should be a passive one, and they should not volunteer opinions, or try to disturb the sessions in any way or manner. Moreover, the product owner should try to limit their numbers during the sessions so it does not become overcrowded and difficult to hold the meeting.

Maintaining a proper approach
It is important to remain focused, and the product owner should be clear whether a particular product backlog item should be estimated again, or it ought to be detailed in greater depth, and additional explanation provided regarding its acceptance criteria. The team members should remain focused upon understanding the PBIs and if required they should demand explanations regarding the acceptance criteria and how the development should be carried out during the sprint activity.