I’ve been using Extreme Programming Planning Poker since James Grenning showed it to me in 2003. Much has been written about it, how to use it, and how it quickly leads to estimates as good as any other more detailed and time-consuming techniques.

I like it when something I use works. I like it even more when I can point to nature or science for the underlying principles of why it works. The sequence of numbers that planning poker is based on came from the Fibonacci set:

“By definition, the first two Fibonacci numbers are 0 and 1, and each subsequent number is the sum of the previous two. Some sources omit the initial 0, instead beginning the sequence with two 1s.

In mathematical terms, the sequence *F _{n}* of Fibonacci numbers is defined by the recurrence relation

F_{n} = F_{n-1} + F_{n-2} with seed values F_{0} = 0 and F_{1} = 1

** Wikipedia**

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This ties back to nature, such as the relative size between the whorls in a nautilus shell, and the golden ratio derived from the summation of the inverse of the set:

where

Source: http://en.wikipedia.org/wiki/Fibonacci_number

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So, faithful to nature, I’ve been using decks of planning poker cards with the numbers 1, 2, 3, 5, 8, 13, 21, 34, 55 …, depending on the size of the product backlog.

Imagine my surprise when I was called by a very distressed Dr. Stephen Hawking, the theoretical physicist. He was inquiring about who was “screwing around” with the Fibonacci sequence. He and his fellows had been noticing a spate of distressing phemona lately, such as:

- The speed of light starting to vary unpredictably.
- Objects falling from trees at varying velocities that were independent of any known attributes.
- Nautilus shells spontaneously bursting apart.
- Other worrisome perturbances of the natural order.

They had studied the problem, and tracked its origins back to us, the agile community of software developers. They found that a number of us were perverting the Fibonacci sequence. They found a number of decks of cards and posters that showed the following sequence:

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Dr. Hawking indicated a high degree of probability that there was a correlation between the abnormal change in natural phenomena and this perversion of the Fibonacci sequence.

Dr. Hawking pleads, and I support, that we cease and desist from our misuse of the Fibonacci sequence, and return to the real sequence. This will reaffirm our relationship with nature and our world.

Thanks for your support.

Best,

Ken Schwaber

Ken, unless my browser is acting the goat, you missed the sequence that was found on decks of cards and posters…

Are they Mountain Goat Software cards?

Yes and Microsoft, with trademarks of the perversion.

+1 to not being able to see the rogue sequence

Also, as a comment on the intent of the article, assuming he is talking about MG’s card point sequence or similar, I think this falls into the “Of all of the obstacles to Scrum’s widespread success, I would hope that Ken Schwaber has bigger fish to fry than the abuse of the Fibonacci sequence.”

Ken, I love ya and all, but we’ve got bigger battles to fight!

Absolutely, just a random brain whiz.

Ken

Planning poker was not designed. It happened, in a pragmatic moment in a stalled meeting. It’s good to reflect back on how it started, how it evolved, and what works.

James

Hi Ken

I’m pretty sure there is no law of software physics that says estimates should grow by this coefficient:

Back in the old days when planning poker was played with note cards, we just wrote the number on the card. To show that there is more uncertainty in larger estimates, the gaps between the numbers naturally grew. Fibonacci was safe from the software developers. Mike Cohn started promoting Fibonacci. You have to print something on the cards, why not Fibonacci? I think it has a small problem for software estimation…

It makes the larger estimates appear to have two significant digits. When I use Planning Poker (for large batches of stories, I usually use an affinity approach) I use the usual numbers up to 8, but once over 8, I tend to use numbers 10, 15, 20, 40,… as far as necessary. The actual numbers don’t really matter too much (to me). A group of stories with the same estimate are not exactly the same, but the estimates are close that number.

I suppose this depends on your scale, but rarely is it wise to work on a story that has two digits. A 10 means we don’t understand the story very well, and uncertainty grows with the estimate. To work on double digit stories, you have to split them. If I need a quick estimate of the whole backlog, it’s a lot easier to ad numbers divisible by 10, at least for me🙂

James

Thanks for your information, which should be the most telling.

Agree with James here…

Having estimates that appear to be more accurate than they are conveys the wrong message. Also, why is mapping to a mathematical series important at all?

Also, I’ve found that 13 is the biggest sized story any team has been willing to pull into a single sprint. Above that, the story sizing is all for backlog sizing and planning (so being able to see “10, 20, 40…” number in that are clear visual indications that the team hasn’t tackled these “in anger” yet.

We don’t have “4” on our cards, but there are times when the team said “yeah, it is 2x that 2pt story” Is it really worth -any- time trying to convince the time that “3” or “5” are the only acceptable votes? The damage for putting a 4 on the story is what exactly? Will the team earn some merit badge or get a bonus from the Scrum Alliance for this? With no upside, any time spent forcing this is time better spent understanding and discussing what the Product Owner wants or how it will be implemented.

-al

Sigh… mea cupla…

Should have realized this was more joke than serious with the Hawking refernce.

I owe someone a beer…

-al

I had a🙂 in my tweet. Does that mean you owe me a beer😉

Things like Fibonacci become part of the dogma, the cargo cult. You’ll find the agile planning tools have it baked into their drop down lists as if it were a law of software physics.

thanks again Ken for the fun and creative post.

James

All,

I think it is important, every once in awhile, to take a step back and recognize that we don’t always need to take ourselves so seriously. And sometimes it is okay to just have a little fun.

Thank you Ken for reminding us that, in the end, we should enjoy what we do, and hopefully have a little fun while doing it.

-AA

I agree it is a fun post. Getting Dr. Hawking involved is great.

thanks again Ken!

James

I guess I’m a buzzkill because I totally didn’t get the Hawking thing, still really don’t. I mean, when I first saw the post, I thought Ken accidentally posted it on 3/11 instead of April fool’s day(when there is no need to put a winky face on something). I assumed his data(observations in nature) was a joke, but there was not much to really communicate that in the post. No winkies, etc. Maybe the Steven Hawking thing was it and I just totally missed it.

Or alternatively, in the spirit of keeping it simple, I often use currency. 1,2,5,10,20,50,100 works just as well for velocity and keeps the Fibonacci confusion away from the non-scientists amongst us.

@Julie – Interesting … never thought of currency as a scale before. Instead of “planning poker”, the estimation exercise can be called “Show me the money !”. I once worked with a team that had a scale of .25, .5, 1, 1.5, and 3. So coins could potentially be in the offing as well🙂 (OK – so there are no 1.5 and 3 dollar notes)

I like the Fibonacci scale myself, but prefer that, if you call it that, the numbers be accurate to the term. I understand the false sense of accuracy that “21” and “34” may convey. Maybe it’s the anal retentive mathematics gene in me, but if you change it, just give it another name. Pseudo-Fibonacci ?

Mike Cohn calls the sequence he uses on the cards a modified fibonacci sequence. I just call it numbers with gaps as the estimates get bigger.

James

I call it something changed so it could be trademarked. Not open source, exactly.

Long before we thought of using the Fibonacci sequence we used linear scales like t-shirt sizes or a range of jelly beans from one to ten. I tend to side with Cohn’s original intent for using a non-linear scale (hey, why not a logarithmic scale?) to help disconnect estimates from hours and get teams into relative weighting without the bias and bickering over what distinguishes a five vs. six jellybean story.

I concur with al’s observation that a “13” point story tends to be the highest that a team feels comfortable in bringing into a Sprint. It’s also why I decided to take Ken’s advice (offered during my PSM I in September) to shorten my scale to:

0, 1, 2, 3, 5, 8, 13, 21

I use “20” right now because I have a deck of those perverted cards from VersionOne that goes 20, 40, 100, infinity.

I had a laugh on this post, tho. Great read.

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Nice.

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One of the advantage of the fibonacci sequence is that it can hardly be copywrited, and therefore you don’t need an authorization to use it.

Surprised more and more how many people seem to be in the business of getting their estimates right, rather than focusing on delivering value. And among those in the business of “getting our Estimates right” (and “what if our estimates are wrong”) a large group seems to be ‘masters’ who think The right way to do that is with Planning Poker — (capital PP with fibonacci or other series).

I am still convinced that “if we estimate” fits agile teams better than “when we estimate”. If we estimate then planning poker (Grenning game) is a great way to help find assumptions, uncertainties. Planning Poker capital P seems to try to turn that into a science, with averaging and standard deviations…

Fun post, Ken, Thanks

Great post – thanks Ken. What a coincidence that I encountered my first rogue deck like this yesterday.

At first I thought I was going mad or my memory had failed me in how the Fibonacci sequence worked. Then I realised this was a bit of a return to the old school of estimation – when a project manager came up with a number. The project board then sent him/her back find a smaller one. I then realised these same people had managed to doctor the cards to ensure reduced estimates. Imagine how much time you could save on a large project by forcing people to choose 20 when they really meant 21😉

Next time I read a blog, I hope that it won’t fail me just as much as this one. After all, Yes, it was my choice to read through, however I genuinely believed you’d have something interesting to talk about. All I hear is a bunch of crying about something that you can fix if you were not too busy searching for attention.

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