The story point is high level estimation based on complexity before the work begins on story while the hour based estimation is just more concrete estimation where effort is represented in hours. The amount of work a team can accomplish during a single sprint is called Velocity. It is calculated at the end of Sprint by adding all the story points which are done-done.
Velocity is a measure of the amount of work a Team can tackle during a single Sprint and is the key metric in Scrum. Velocity is calculated at the end of theSprint by totaling the Points for all fully completed User Stories
Now the question is how does story points relate to time and this is certainly one of the debated topic among sprint teams and I see confusion all around. I heard from individual team members many a times that story point can never be related to time in any way. One cannot say 1 story point equivalent to 2 hours or 5 hours or any other number. The second statement is right while this creates a notion that story point has no relation with time. When you have 3 story points, it could take 3 days or up to 50 days. This doesn’t seem right and this is actually not RIGHT. “Whatever time it takes is fine after-all one is putting on effort” effect is what visible .
The reality is, the story point states EFFORT. The estimation for team member X can be 10 hours for 3 story points while other it could be 20 hours. The moment you say effort, it is size. But the size is relative. Hence it proves the point that it is not exact hours while it’s relative hours. The estimate is for team and they should be able to say what is the biggest size in terms of story points can be picked which would be done in sprint. Moreover if we say, team velocity is 40 story points that means we can complete 40 story points for this team in N week sprint.
If one doesn’t want to relate this to size at all, then I suggest them to stop doing estimation as in that it case it would be just waste of time. Typically a story point suggest min and max time which might be taken to accomplish that story. For instance 3 story point for team P means effort between 3 and 5 days. It could be plus or minus but the point I have is to have some indication with respect to size.
I have seen individuals just change the story point for a given story if they can’t complete the same in stipulated time. I understand the estimation was not done right or probably we might not have done estimation but adding/changing story point or hours post the work is being done is pathetic. It completely wastes the time spent on estimation and further we lose all the value gain with estimation. This practice is primarily meant for inflating numbers to have better reports. This MUST be stopped.
I have been often asked that the team is not able to predict well because they are new or may not be competent enough to find the right story points. How should we handle that as it is important to be predictable as you can’t run the business in vacuum? When your boss or business ask how long it takes to complete the effort, you still have to answer probably by looking at release planning and estimation. This shows another point why we have to relate the effort to hours (Not the exact) and here min and max does the job.
As we are talking about story points, there are many teams who would just add story points to story while for individual tasks they don’t assign hours. The burndown chart in that case is driven by task (Task based or hour based) which is in a way weird because you suddenly see ups and downs. The whole value of one of the most critical artifact in agile is not being materialized. The planned and remaining hours is important in individual tasks.