story points

1+3 != 4

Please stop adding story points

“I’m not sure if I was the inventor of story points, but if I am, I’m sorry” – Ron Jeffries

What is a story point

At their heart story points are an estimate of how much effort/complexity a piece of functionality is to deliver. This estimate is often represented as a number.

That’s fine, but please stop adding estimates!

Let’s say you use Fibonacci numbers to define your story points;

  • A 1 point story could be a trivial change, like fixing a typo
  • A 3 point story could well be something more involved, say adding a couple of textboxes to a website page, and persisting the data entered in them.

How much work is that?

If you have 2 stories, one estimated at 3 points and one at 1 point, how many story points is that? A total of 4 points is something I have heard.

Now, what if I have 10 stories, all sized at 1 point. By this logic we have a total of 10 story points. Nothing wrong with this, assuming the only goal for adding the story points is to re-affirm en elementary grasp of numbers. It gets funny if… you want to know how long the project will take to complete, based on this.

Week 1, we complete only 3 point stories, and complete 5 of these in 5 days.

Week 2, we complete only 1 point stories, and complete 100 of these in 5 days.

I have seen this interpreted as, in 10 days, 105 story points delivered, averaging a velocity of 52.5 story point per week. The sized backlog, stands at 150sp, so all should be complete in approx 3 weeks.

Spread the points

mathsHopefully you can see the problem here.  We can deliver small stories really fast and bigger stories, slow.

In the above example, the longest 3 point story took 2 days to deliver. The longest 1 point story took 3 hours.

The shortest 3 point story took 3 hours and the shortest, 1 point story, took 0.5 hours.

The backlog, has 99 one point stories and 17 three point stories (150 story points, mentioned earlier), so the longest time to complete could be:
99*3hrs (297) + 17*12 (204) = 501hrs = 83 days.

Optimistic time is:
 0.5*99 (50) + 17*3 (51) = 101hrs  = 25 days

At this stage I wonder if I’m writing a maths blog… Lots of numbers here, all based on, an estimate! Some people make the mistake of aiming to deliver n story points per sprint, and when stories are not delivered post mortems happen, to see what went wrong.

“I smell something fishy and I’m not talking about the contents of Baldrick’s apple crumble.” – Blackadder

Nonsense, the lot of it. The only way you can deliver a consistent number of story points per iteration, is if all those stories are the same size and your team’s ability to estimate (and create stories) that are the same size is amazing.

Keep it simple

If every story is enhancing your product (as they should), at the end of your iteration some stories will be ‘done’ and you will have enhanced your product. Story points can be used for getting a feel for the size (complexity/amount of work) a story is. This is useful to the product owner to see how much effort a requested feature is, to help with their prioritisation. It’s also useful if you want to ensure you only take stories of a certain size into your iteration (i.e. a max of 3, say), encouraging story splitting. Story points have other benefits too, but for ‘big’ releases and tracking story point burn up/down/velocity… please stop.

In football the only statistic that matter is the score line; in software, it’s what’s delivered.

Image Credit: Pranav Yaddanapudi