Planning and estimating tasks in Scrum involves creation of User Stories, approval and estimation of User Stories, creation and estimation of tasks, and creation of Sprint Backlog. Creation of User Stories involves writing of User Stories and their related User Story Acceptance Criteria. User Stories are usually written by the Product Owner and are designed to ensure that the customer’s requirements are clearly depicted and can be fully understood by all stakeholders. The Product Owner, based on his or her interaction with the stakeholders, business knowledge and expertise, and inputs from the team, develops User Stories that will form the initial Prioritized Product Backlog for the project. The Prioritized Product Backlog represents the total sum of what must be completed for the project. The objective of this exercise is to create elaborated and refined User Stories that can be approved, estimated, and committed to by the Scrum Team. At times, the Product Owner may bring a Business Analyst to assist with writing User Stories. User Story Writing Workshops may be held for creating the User Stories.
Although the Product Owner has the primary responsibility for writing User Stories and often carries out this exercise on his or her own, a User Story Writing Workshop can be held if desired. Approval for User Stories of a Sprint is provided by the Product Owner. Then, the Scrum Master and Scrum Team estimate the effort required to develop the functionality described in each User Story. Finally, the Scrum Team commits to deliver the customer requirements in the form of Approved, Estimated, and Committed User Stories.
The approved, estimated, and committed User Stories are broken down into specific tasks and compiled into a Task List. Often, a Task Planning Meeting is held for this purpose. In Task Planning Meetings, the Scrum Team gets together to plan the work to be done in the Sprint. The team reviews the committed User Stories at the top of the Prioritized Product Backlog. The Product Owner is present during this meeting in case clarification is required related to User Stories in the Prioritized Product Backlog and to help the team make design decisions. To help ensure that the group stays on topic, this meeting should be Time-boxed, with the standard length limited to two hours per week of Sprint duration. This assists in preventing the tendency to stray into discussions that should actually occur in other meetings, like the Release Planning or Sprint Review Meetings. By the end of the meeting, the entire Scrum Team will have fully committed to deliver a subset of User Stories from the Prioritized Product Backlog in the Sprint.
Here is a video on planning and estimation in Scrum: http://www.scrumstudy.com/watch.asp?vid=611
Then the Scrum Core Team, in Task Estimation Meetings, estimate the effort required to accomplish each task in the Task List. Task Estimation Meetings enable the Scrum Team to estimate the effort required to complete a task or set of tasks and to estimate the people effort and other resources required to carry out the tasks within a given Sprint. In Task Estimation Meetings, the Scrum Team members use the Task List to estimate the duration and effort for the User Stories to be completed in the Sprint. One of the key benefits of this technique is that it enables the team to have a shared perspective of the User Stories and requirements so that they can reliably estimate the effort required. The information developed in the Task Estimation Meetings is included in the Effort Estimated Task List and it is used to determine the velocity for the Sprint. In this workshop, the Scrum Team may use various techniques such as decomposition, expert judgment, analogous estimation, and parametric estimation. The result of Task Estimation Meeting is, often, an Effort Estimated Task List.
And finally, the Scrum Core Team holds Sprint Planning Meetings where the group creates a Sprint Backlog containing all tasks to be completed in the Sprint. During Sprint Planning Meetings, the User Stories, which are approved, estimated, and committed during the Approve, Estimate, and Commit User Stories process, are taken up for discussion by the Scrum Team. Each Scrum Team member also uses Effort Estimated Task List to select the tasks they plan to work on in the Sprint, based on their skills and experience. The Scrum Team also creates the Sprint Backlog and Sprint Burndown Chart using the User Stories and the Effort Estimated Task List during the Sprint Planning Meetings.