Ideally a Sprint in SCRUM is assumed to be successful when the team has delivered all the user stories which were committed to during the Sprint Planning meeting. But in real life scenarios, amidst lot of uncertainties and unpredictabilities, this is far from being achievable.
Due to this, the definition of a successful Sprint does not necessarily mean that every single User Story on the Wall has to be moved to the Finished/ Complete Status.
A sprint can be called successful in any of the following cases:
1. About 80 % of the committed stories are complete,
2. Tasks / things carried over to the next sprint are limited and can be finished very quickly.
3. Atleast 50% (or some, figure depends on the type and content of the user stories) of the stories are complete. In other words, if ALL the stories are in progress at the end of the sprint and NONE are complete (even if 80% of committed work is complete), it can’t be called as successful sprint.
Most of the SCRUM community believes that a 100% success rate for sprints is impossible to achieve and if that happens quite often, then there’s something fishy!!