In Chapter 3.2.2 in Agile Tester Extension: Estimating Testing Effort Based on Content and Risk

The agile project test strategy or test approach (the distribution of the test effort and coverage over the items to be tested) is defined during the planning event and included in the backlog. For example, during the planning stage, the agile team estimates, with the aid of planning poker, the size (often estimated in story points) for each story of a product backlog. Planning poker is the ideal way to formulate relative estimation size; i.e., where the estimations are related to one another. A reliable estimation is made by the whole team. By means of cards, everyone allocates story points to the estimated size of a backlog item. Aspects such as effort, complexity and the thoroughness of testing (in relation to the product risk) play a role in the estimation. Therefore, it is advisable to include the risk classification of a backlog item - particularly in the case of user stories - in addition to the priority specified by the product owner, before the planning poker is initiated. Differences in estimates are discussed, after which the card-playing is repeated until consensus arises. The discussion that produces this means of evaluation ensures that nothing is forgotten and that everyone is involved. This ensures a reliable estimation of the work — across the various disciplines — which is needed to complete a product item and to boost collective knowledge of what has to be done.

See also Kelly Waters article about this topic:

http://www.allaboutagile.com/estimating-in-agile-development/

The text above is a sample from the upcoming Agile Tester Extension that will be released in early 2014. Please note that Agile Tester Extension is in its alpha phase, which means that its content may change. Visit www.istqb.org to get latest information.