Although all work may not be completed within a Sprint we want to avoid unfinished work at the end of a Sprint as much as possible.  It is desirable that Product Backlog Items (PBIs) assigned to a Sprint are completed within that Sprint. When it is evident this will not be the case, we should alter our plans.
Sprints are designed to deliver value . When Stories are incomplete, no value will be delivered and no Increment will have been created
Increments of value in the Sprint help achieve the Sprint Goal. If there are incomplete Stories, then we'll fail to achieve the Sprint Goal.
If no Stories meet the Defintion of Done, then there will be nothing to review and get feedback on.
Without an Increment of value at the end of the Sprint, the Sprint Goal will not have been meet and progress toward the Product Goal will be unknown.
Sometimes, a Product Backlog item while still not complete, a part may be of value.
Team members should be inspecting progress toward the Sprint Goal on a daily basis at the Daily Scrum (“Stand-up” if the team is doing Extreme Programming). This provides the team the opportunity to:
Increments of value in the Sprint help achieve the Sprint Goal. If there are incomplete Stories, then we may fail to achieve the Sprint Goal.
Have any items met the Definition of Done? What is needed for items to achieve the Definition of Done today?
Accomodate any adjustments to the team's plan for the day to achieve the Sprint Goal by adjusting the Sprint Backlog.
Ensure that Stories are assessed in terms of meeting the Definition of Done at Daily Scrum, and any adjustments made, to ensure value is delivered and the Sprint Goal is achieved. Focus on the state of Increments and the Sprint Goal, not on going "around the room" and asking everyone for a status report on their tasks.
Use retrospectives for the team to reflect upon why a Story could not be completed within the Sprint. Common causes for incomplete Stories include:
1. Cohn, M. (2014) Unfinished Work at the End of a Sprint is Not Evil. https://www.mountaingoatsoftware.com/blog/unfinished-work-at-the-end-of-a-sprint-is-not-evil2. Sutherland, J. & Schwaber, K. (2020) The 2020 Scrum Guide (TM). https://scrumguides.org/scrum-guide.html3. Cohn, M. (2015) Handling Work Left at the End of a Sprint. https://www.mountaingoatsoftware.com/blog/handling-work-left-at-the-end-of-a-sprint
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