The exit-criteria to determine whether a product backlog item is complete. In many cases, the DoD requires that all regression tests be successful. The definition of done may vary from one scrum team to another but must be consistent within one team. The total effort a team is capable of in a sprint. The number is derived by evaluating the work (typically in user story points) completed in the last sprint. The collection of historical velocity data is a guideline for assisting the team in understanding how much work they can likely achieve in a future sprint. A time-boxed period used to research a concept or create a simple prototype. Spikes can either be planned to take place in between sprints or, for larger teams, a spike might be accepted as one of many sprint delivery objectives. Spikes are often introduced before the delivery of large or complex product backlog items in order to secure budget, expand knowledge, or produce a proof of concept. The duration and objective(s) of a spike is agreed between product owner and development team before the start. Unlike sprint commitments, spikes may or may not deliver tangible, shippable, valuable functionality. For example, the objective of a spike might be to successfully reach a decision on a course of action. The spike is over when the time is up, not necessarily when the objective has been delivered. Also called a drone spike, a tracer bullet is a spike with the current architecture, current technology set, current set of best practices that result in production quality code. It might just be a very narrow implementation of the functionality but is not throwaway code. It is of production quality, and the rest of the iterations can build on this code. The name has military origins as ammunition that makes the path of the bullet visible, allowing for corrections. Often these implementations are a 'quick shot' through all layers of an application, such as connecting a single form's input field to the back-end, to prove the layers connect as expected. The benefits of Scrum may be more difficult to achieve when:   From a business perspective, Scrum has many virtues, one of which is that it is designed to yield the best business solutions. However, the efficiency by which it does so in any given organization can vary widely and is largely dependent on the ability of the organization to adhere to the implementation guidelines. Every company has its own distinct organizational structure, culture, and set of business practices, and some are more naturally amenable to this methodology than others. Like other agile methods, effective adoption of Scrum can be supported through a wide range of tools.