Why estimating in Story points is better than estimating in hours
Posted on July 8th, 2009 in Agile by siddharta || No Comment
Doing Distributed Agile?
Here are some reasons why Story point estimates are more useful than estimating in actual time (hours or days):
- Story points represent a probability distribution of hours, not a fixed number of hours. That is to say some 1 point stories may take a day, some may take two days, some may take half a day.
- Story points are independent of the people implementing the story. An experienced developer may take 2 days for a 3 point story, but a new developer may take 5 days for the same. Thus, you do not have to re-estimate the complete backlog if a person leaves, joins, gets ill, or goes on leave.
- Estimating in hours has the problem of not forecasting non-working hours. For example, if I say a story will take a 8 hours (a day), but there happens to be a meeting on that day, then it may actually end up taking two days because I spent my time in the meeting. To estimate accurately in hours, you need to be able to forecast the distribution of non-working hours. These kind of non-work hours are random and difficult to calculate up-front.
Here is a simple example:
Lets say we have two stories scheduled for this week. One is 1 point, another is 3 points. Let us also say that we have one day of meeting.
Scenario 1
The meeting fell when I was working on the 1 point story – so it ended up taking 2 days to do. I finished the 3 point story in 3 days. So in the week, I managed 5 points.
Scenario 2
The meeting fell when I was working on the 3 point story – so it ended up taking 4 days to do. I finished the 1 point story in 1 day. So in the week, I managed 5 points.
In both cases, the total points completed is the same – 5 points. But the days taken to complete each story is different.
By estimating and scheduling in points, we are able to average out the effects of random variations in non-work hours. But to estimate accurately in hours or days, I need to know beforehand on which day the meeting is going to fall. Otherwise the breakdown becomes wrong.
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