Estimation by definition is choosing a value using the best information available. The value needs to take into account the uncertainty involved. Keeping in line with the principle of Self Organization, the Guide to Scrum Body of Knowledge™
recommends the usage of Planning Poker to estimate tasks and other items. The usage of Planning Poker or Scrum Poker in the context of the Task Estimation Meeting is discussed below.
Planning Poker is a structured team communication mechanism to arrive at estimates by consensus. The Scrum Master acts as the moderator of the Task Estimation meeting. The Product Owner maybe present as an observer. All the participants are supplied with Fibonacci series or currency like cards that can be used to represent any number up to a certain value. A task or item that is well understood by all the participants is discussed, selected as the base task and a certain value is assigned to it. It is recommended that the value assigned be in terms of an abstract unit like story points or carrots.
Then each task from the Task List is taken up, discussed and compared to the base task. Then each participant individually values the task by placing the appropriate cards covered in front of him/her. Then all the participants uncover their cards and identify the lowest and highest bidders, who will argue their standpoints before repeating the estimation card process till consensus is achieved.
Planning Poker balances the problem of balancing group thinking vs individual thinking by having individuals take a stand before every participant’s stand is announced to all, thus avoiding the problem of anchoring. Also it uses relative estimation rather than absolute estimation due to the tendency of humans to be better at the former. Planning Poker is usually classified as a variant of the wide-band Delphi method