When to re-estimate? What is a better way to estimate : Story points or Ideal Days?

When to Re-estimate?

Story points and ideal days are estimates of the size of a feature which helps you to know when to re-estimate. Re-estimate is done only when your opinion of the relative size of one or more stories has changed. One should not re-estimate just because progress is not coming as rapidly as expected.

Velocity should be allowed to take care of most estimation inaccuracies. Velocity is considered to be a great equalizer. The reason behind this is that the estimate for each feature is made relative to the estimates for other features, it does not matter if our estimates are correct, a little incorrect, or a lot incorrect. What matters is that they are consistent. As long as we are consistent with the estimates, measuring velocity over the first few iterations will allow us to have a reliable schedule.

At the end of an iteration, it is not recommended giving partial credit for partially finished user stories. The preference is for the team to count the entire estimate towards their velocity (if they completely finished and the feature has been accepted by the product owner) or for them to count nothing toward their story otherwise.

However, the team may choose to re-estimate partially complete user stories. Typically, this will mean estimating a user story representing the work that was completed during the iteration and one or more user stories that describe the remaining work. The sum of these estimates does not need to equal the initial estimate.

A team can choose to estimate either through story points or ideal days. Each has its advantages.


Benefits of Story Points
1. They help drive cross functional behavior.
2. The estimates derived by story points do not decay.
3. Story points are a pure measure of size.
4. Estimation through story points is faster.
5. Unlike ideal days, story points can be compared among team members. If one team member thinks that it will take him 4 ideal days, and another member thinks that it will take him 1 ideal day, both of them may be right yet there is no basis on which to argue and establish a single estimate.

Benefits of Ideal Days
1. They are more easily explained to those outside the team.
2. They are easier to get started with.

The advantages of story points are more compelling as compared to benefits of ideal days. one way is if a team is struggling with estimating the pure size, they can start off with estimating with ideal days and gradually switching to estimating by story points.