All the entries within the Scrum Product Backlog have to be estimated to allow the Scrum Product Owner to prioritize the entries and to plan releases. This means that the Scrum Product Owner needs an honest assessment of how difficult the work will be. Nevertheless it is recommended that the Scrum Product Owner does not attend the estimation to avoid pressuring (intentionally or otherwise) the Scrum Team.
The Scrum Framework itself does not prescribe a single way for the Scrum Teams to estimate their work. However within the Scrum Framework the estimation is not normally done in terms of time - a more abstracted metric to quantify effort is used. Common estimating methods include numeric sizing (1 through 10), t-shirt sizes (XS, S, M, L, XL, XXL, XXXL) or the Fibonacci sequence (1, 2, 3, 5, 8, 13, 21, 34, etc.).
Important is that the team shares a common understanding of the scale it uses, so that every member of the team is comfortable with it.
Planning Poker® / Scrum Poker
One commonly used method during the estimation process is to play Planning Poker® (also called Scrum Poker). When using Planning Poker®, influence between the participants are minimized and therefore a more accurate estimation result is produced.
In order to play Planning Poker® the following is needed:
The list of features to be estimated
Decks of numbered cards.
A typical deck has cards showing the Fibonacci sequence including a zero: 0, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89; other similar progressions are also possible. The reason for using the Fibonacci sequence is to reflect the uncertainty in estimating larger items. A high estimate usually means that the story is not well understood in detail or should be broken down into multiple smaller stories. Smaller stories can be estimated in greater detail. It would be a waste of time to discuss if it is 19, 20 or 25; the story is simply (too) big.
The game is then played in the following steps:
The Scrum Product Owner presents the story to be estimated. The Scrum Team asks questions and the Scrum Product Owner explains in more detail. If many stories have to be estimated a time-constraint (e.g. only one minute for explanation) might be set as well. If the time-constraint is reached and the Scrum Team does not understand the story it is a sign that the story has to be re-written.
Each member of the Scrum Team privately chooses the card representing the estimation.
After everyone has chosen a card, all selections are revealed.
People with high and low estimates are allowed to explain their estimate.
Estimation starts again until a consent is found.
This game is repeated until all stories are estimated.
Planning Poker® is a registered trademark of Mountain Goat Software, LLC.