Accountability

That's right. The entire Scrum Team is accountable for creating a valuable, useful Increment every Sprint. The team therefore needs to ensure work is ready for Sprint Planning.

The Product Owner is also accountable for effective Product Backlog management, which includes:

• Developing and explicitly communicating the Product Goal;

• Creating and clearly communicating Product Backlog items;

• Ordering Product Backlog items; and,

• Ensuring that the Product Backlog is transparent, visible and understood.

I don't agree in my opinion the product owner is accountable for refinement. 

The Developers who will be doing the work are responsible for the sizing.

Abstracting from the above thinking, is the fact that the part of refinement is sizing not enough to put refinement outside of single po accountability ?

Abstracting from the above thinking, is the fact that the part of refinement is sizing not enough to put refinement outside of single po accountability ?.

No not in my opinion. see the definition of accountable and responsible below.

The accountable person is the individual who is ultimately answerable for the activity or decision. ... Only one accountable person can be assigned to an action. The responsible person is the individual(s) who actually complete the task. The responsible person is responsible for action/implementation.

So the product owner is accountable for refinement including the sizing. the developers are responsible for the sizing.

One thing I want to point out is that the Scrum Guide does not list individuals or job descriptions. It provides 3 distinct roles.  Each role can be done by a wide variety of job titles.  With that in mind, I would like to point out that @Sven Lankreijer's definition of accountable and responsible is focused on individuals.  While that is correct, in context of the Scrum Guide I would suggest looking at accountability and responsibility in relation to the 3 roles, not a single individual.  Otherwise on anything for which the Developers are accountable, how would you decide which of the Developers is the one responsible? 

So my opinion is that the entire Scrum Team is responsible for refinement  since everyone has a role to play. The Product Owner provides initial statement and clarifying information, the Developers break the item into smaller, more manageable statements of work, the Scrum Master helps the team understand the benefit and importance of doing the refinement work. 

The accountability lies with the Developers role since they are ones that execute the work so the rest of the team relies on them to provide workable units of a size that are precise enough to deliver value. 

In Scrum, accountability may also lie with the entire scrum team as a group of people, not only developers as a group of people. Giving accountability to a team is consistent with collective intelligence. I agree with Daniel when it comes to sizing and maybe even precise pbis (then developers), but ordering is also part of refinement (then po). Then scrum team

As the steward of the PO class, I always say the product owner makes sure the product backlog is refined.  Backlog refinement can happen with the rest of the scrum team, stakeholders, subject matter experts, or even just the product owner alone.  By the time a PBI is ready to be planned though, nobody should be surprised and you should have had the whole scrum team refine the PBI.

I don't recommend the pattern - write the PBI, refine once, plan the PBI.  I recommend refining a PBI 4 - 6 times before it's planned but each refinement is just a little bit - so that you don't add too much detail too soon or too little detail too late.

How do you know how much detail - purposefully ask this question at the retrospectives.

Chuck, does it mean PO is accountable for the act of refinement?