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 is 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.