I understand that on the first part of your sprint planning, you would size the stories for example using story points and poker cards and then you estimate how many stories you can tackle on the next sprint. So let´s say you agree on working on 3 stories with a total combined of 18 story points.
Then you go ahead with the second part of the sprint planning and you start breaking those stories down into tasks. These tasks are now estimated using actual hours.
I have two questions:
1) How do you estimate the tasks in hours? do you use again poker estimating but with number of hours this time? how does the team agree on the number of hours for each task.
2) Once you estimate all tasks for the 3 stories you agreed for the sprint, you find out that the number of total hours combined for all tasks of all 3 stories is 90 hours for example. If your actual team capacity in hours is 75, how do you adjust your initial commitment of delivering 3 stories now that you realize that you don´t actually have the time to do it? Do you go back to your Product Owner (if he´s not there anymore) and tell them that you will be delivering 2 stories instead or how do you go about this?
Thanks a lot in advance for your help!
The tasks are not really estimated in hours but ideal hours. It's really hard to predict how many ideal hours will be available in a week, and it's generally not a good idea at all to infer the capacity on a sprint based on hour estimates. See for example this Scrum Alliance blog article
Story points and task hour comparison can be thought of as the comparison of the weight and height of an elephant. In general, a taller elephant may be heavier than a shorter one, but this is not always the case. There is no biological proof of a weight-versus-height formula, despite the common perception that more height means more weight. The same explanation applies to story points versus task hours: In general, a more complex user story (higher points) should take more hours to complete, but there are always exceptions.
Normally, tasks are tackled by a single team member (or a pair), and thus they are estimated by them, and not by the whole team.
Furthermore, tasks are re-estimated daily: the number we look for is the number of ideal hours left to complete the task and not the total amount. So, it is a number that can go up or down, or remain constant.
Also, tasks can be added or removed during the sprint. It's actually pretty common to discover that initial plans change. It doesn't matter, as long as the total number of hours planned represents the best current estimate of what's left to do -- it's needed for burndown charts.
In conclusion, don't mind the hours. Use them to monitor progress during the sprint but not to determine capacity. This is what points are for and the two parameters are not interchangeable as it would seem.