Developers make estimates because you have the most insight and understanding about the tasks being targeted. Estimates are made in rooming sessions. A product owner rooms this estimate. The Scrum Master organizes the meeting. The Rooming Session should take a maximum of 10% of the developer time. Estimates are an approximation in hours. Estimates will be post-corrected over the course of the sprints and estimates will be optimized.
Estimates need to be trained. A reference story or task area is used. One takes an easy to implement story that is clear, simple and known to everyone. By Triangulation - Planning Poker an estimate is made. Each developer makes an estimate on the reference story and determines a number on how high they estimate the effort will be. After everyone has estimated, the highest and lowest estimates are discussed. This is followed by agreement on a number or category for the requirement. A Fibonacci number, T-shirt size, or categories such as Very Light - Light - Moderate - Rather - Heavy - Very Heavy - Extremely Heavy can be used. Effective time can also be given in ideal hours.
The planning poker meeting should not be done in the sprint planning. The planning poker can be done at the initial backlog creation or at a backlog grooming meeting where requirements from stack holders are incorporated into the project. All members of the development team should be present. The Scrum Master or the Product Owner moderates the meeting and explains the rules. The moderator introduces each user story. Each team member, after estimating, places the card upside down on the table so that other team members are not influenced. It should be estimated harder rather than easier. After the scrum team estimates the effort, the cards are flipped over simultaneously. The players with the highest and lowest estimate must justify the decision. The Scrum team discusses the estimates objectively and constructively for up to two minutes. The planning poker is then played again until an agreement is reached. The estimates of the participants who implement the tasks are weighted higher. The Planning Poker allows for collective decision making on the effort of the tasks, transparency and clarity on the details of the tasks. Participants should estimate with sufficient energy and take breaks as needed.
Affinity estimation is sorting user stories or tasks by size or effort. After estimating user stories, they are sorted into groups. Story Points are used for Velocity or Work Speed. The average number of Story Points after a Sprint provides information about how fast the team is working.