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In Scrum, the owner of the estimates is very clear. Only the people who do the work, i.e., the Team, have the right to offer estimates for the Product Backlog and Sprint Backlog items. However, like many things in Scrum, the framework leaves it up to each Team to decide on what is estimated during planning. Because Scrum Teams are self-organizing, they can choose to estimate duration, effort, size, complexity or anything else that makes sense to them.
When I work directly with teams, I prefer the Team to estimate the size of user stories using story points rather than the duration or the effort. Jeff Sutherland, co-creator of Scrum, is also a big proponent of story points in this article on why story points are better than hours. Here are my reasons why.