The Product Owner brings Product Backlog Items to the Scrum Team to estimate their effort (cost). In order to create the right environment of safety and accountability, no Product Backlog Item is estimated by a single member of the Scrum Team, or even a subset of the team membership.
By having all the members of the Scrum Team participate in the estimation work for every Product Backlog item, it becomes impossible to blame a single Team Member for a poor estimate. At a practical level, it should be very rare that a single Product Backlog Item is fully implemented by a single Team Member. Therefore, estimates should consider the collective effort of the Scrum Team, and this can only be determined by having all the Team Members participate in the estimation work. If the team delegates estimation to a single person, or if one person dominates the estimation work, the other Team Members will not have ownership of the estimates and will be able to deny accountability. The pressure on the team from collective estimation encourages teamwork, cross-training and these behaviours in turn promote the development of a high-performance team.
Scrum does not define a required method of estimation, nor does it even require estimation except in two ways: to allow a team to determine what work will be completed in the Sprint, and to determine the ordering of Product Backlog Items. Since the requirements for estimation in Scrum are so light, most Agile estimation techniques are similarly light. The most common technique is called “Planning Poker” and involves the Scrum Team using specially printed playing cards to vote on the effort of Product Backlog Items. This method is also called the Planning Game. Detailed instructions can be found here. Another simple technique that is even faster than Planning Poker is referred to as “the Bucket System”. The Bucket System allows for a divide-and-conquer approach to estimation while maintaining collective responsibility for final estimates. It works by physically laying out a row of numbers representing sizes along the edge of a table or at the top of a wall and having the Scrum Team place Product Backlog Items in the various “buckets” to represent their relative effort. There are many Agile estimation techniques available and a Scrum Team should experiment with them. The Scrum Master should ensure that any techniques used prevent any individual responsibility for estimates. Any estimation work done should be part of either the initial preparation of the Product Backlog before the first Sprint or as part of the ongoing Product Backlog Refinement work. Generally speaking, estimation of individual Product Backlog Items should not be done during Sprint Planning, although from time to time a new Product Backlog Item or a split Product Backlog Item may require just-in-time estimation during Sprint Planning. The Product Owner and Scrum Master have no authority to change or directly influence numerical estimates, but they participate in the process through the explanation of the Product Backlog Items and relevant obstacles.
The Development Team is responsible for all estimates. The Product Owner may influence the Development Team by helping it understand and select trade-offs, but the people who will perform the work make the final estimate. — The Scrum Guide
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