Teams that Finish Early Accelerate Faster
... the Team may be doing well but struggles every Sprint to complete the Sprint Backlog. Worst case, the team is feeling demoralized, velocity is low, and Sprint Backlog is not completed Sprint after Sprint.
If the team takes on too much work, they feel overburdened and under pressure. When they cannot get the work done by the end of the Sprint everyone is unhappy – the development team, the ScrumMaster, the Product Owner, and the management. Furthermore, they do not have time to think clearly about how to improve their work and they enter the next Sprint with undone work. They exhibit all the symptoms of a losing sports team.
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Teams often take too much work into a Sprint and cannot finish it. Failure prevents the team from improving.
Teams can be optimistic about their ability to finish requested features. But in doing so, they fail to give themselves time to reduce technical debt and sharpen their saws. Thus they are doomed to a persistently slow pace.
Individuals on a team may be subversively working on other things. This gets hidden in the work of a team that persistently overreaches.
Managers pressure teams because they lack basic understanding of lean practices. Taiichi Ohno’s taxonomy of waste has the category of “Absurdity” — stress due to excessive scope.  This can cause massive slowdown.
Muri (無理) is a Japanese word meaning “unreasonableness; impossible; beyond one's power; too difficult; by force; perforce; forcibly; compulsorily; excessiveness; immoderation,” and is a key concept in the Toyota Production System (TPS) as one of the three types of waste (muda, mura, muri) (see articles for respective terms in Wikipedia).
Maximize your probability of success by using the pattern Yesterday’s Weather. Then implement Illegitimus Non Interruptus that will systematically deal with any interruptions cause you to finish early. On early completion pull forward from the next Sprint’s backlog, which will increase Yesterday's Weather for future Sprints. To increase the probability of acceleration, apply Scrumming the Scrum to identify your kaizen in the retrospective. Put the kaizen in the Sprint Backlog with acceptance tests for the next Sprint as top priority.
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Openview Venture Partners noticed this pattern after analyzing data from many dozens of sprints with multiple teams. The teams that finished the Sprint early accelerated faster.  Surprised, they observed that Scrum is not about velocity, it is about acceleration! Looking for root cause they discovered the finishing early allowed teams to think more clearly about what they were doing, remove impediments, pull forward backlog from the next Sprint, develop a winning attitude, and increase Yesterday's Weather.
 J. Sutherland and I. Altman, “Take No Prisoners: How a Venture Capital Group Does Scrum,” in Agile 2009, Chicago, 2009.
Picture from: M. Smelter, https://commons.wikimedia.org/wiki/File:Women%27s_team_pursuit_2012_Summer_Olympics,_Dutch_team.jpg, (under CC BY-SA 3.0 license).