Agile Estimation

You a member of an agile development team planning out your next sprint.  You have estimate your velocity at 33.  You currently have a load of 32.  There is a remaining story that has been estimated as having 2 story points (using a Fibonacci sequence).  Would it be a mistake to try to fit it?

If you find yourself asking this, you are doing it wrong.

One premise of most agile techniques is that we are really bad at estimating.  Story points do not try to correct that fact, they simply work around it.  Unless you have the gift of psychic clairvoyance, there is no point in attempting high precision estimates because any such estimate will be wrong.  Hence the use sizes that increase either exponentially or through a Fibonacci sequence.  Assuming you were reasonably accurate, that story you estimated at 13 points might be as little as 12 points.  Or it might be closer to 15.

Remember high school chemistry when you learned about significant digits?  Story points are so low precision they don’t even have one significant digit.  And in a calculation involving low precision measurements, claiming a higher precision result is misleading at best.  Its downright fraudulant at worst.

So back to our above scenario, claiming your story point load is “32” is wrong. You don’t have enough precision in your measurements to say that.  In reality, your load without that extra story is better expressed as “around 30”.  And with the extra story, it is also “around 30”.  If your current load is dominated by a couple 13 point stories, those are what will determine whether or not you meet your goal.  If it is dominated by many small 1, 2, or 3 point stories, you are misleading yourself if you argue you can predict exactly how many you are going to finish.

Is my point that you should give up on estimating?  Of course not.  Just don’t obsess yourself with getting all your numbers to line up.  Commit to enough that you feel comfortable with, and then give yourself plenty of stretch goals. That way you can meet your commitment if your estimates were too low, and you will have enough to do the entire sprint if your estimates were too high.  Because all you really know is that it is unlikely your estimates are spot on.


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