Criteria for good tasks: SMART

  • Specific
    • Tasks must be specific.
  • Measurable
    • The completion conditions of tasks must be clearly defined.
  • Achievable
    • It must be possible to accomplish the tasks.
  • Relevant
    • All of the tasks must be relevant to each other.
  • Time-boxed
    • A time box must be set.

Specific

Tasks must be specific enough for everyone to understand and be involved.
Each task must not overlap with others, and the role each task will play in the story must be clear.

Measurable

The most important point here is the status the task must reach before it can be treated as completed.
The development team must agree to these terms.
This could take the form of “operates as intended”, “automated tests are created”, “code is refactored”, etc.

Achievable

The people responsible for the tasks must foreseeably be able to accomplish them.
Other team members can help to accomplish the task.

Relevant

All tasks must be relevant to each other and contribute to the completion of the story they are connected to.
While stories are allocated for the developers, it must be possible to explain and justify all tasks to the client.

Time-boxed

Time boxes need to be set for tasks. These are not official estimates, but are necessary in order to determine when to ask for help from the team.
It is important to be aware that if a task is more difficult than expected, the team needs to divide it up, assign it to a different person or do something to help.

Source: http://xp123.com/articles/invest-in-good-stories-and-smart-tasks/