Sunday, 15 April 2012


Think of a number between one to four.

100 years ago the Titanic sank.

When the Cameron film first came out I was cycling around County Cork, Titanic's last port of call. Very popular apparel were t-shirts with the slogan:

"The ship sank. get over it."

I was reminded of this today when one of my colleagues posted it on Facebook. It also reminded me of a capacity management class exercise we used to do on the ITIL v1 courses.

Imagine, and I know this will take some imagination, that the unsinkable SS Itil is sailing towards America wth the class of 20 people on board.  Remember this was a long time ago when ITIL hadn't conquered America. Unfortunately en route it runs into an iceberg and starts to sink.

Don't worry though. Those clever people who developed ITIL didn't make the same mistakes as the White Star line, and  there are four life boats, each able to carry 5 people, so enough for all 20 people in the class. More worrying though is that the boat is sinking quicker than expected and you've only got time to get to the first lifeboat you chose. Remember that number I asked you to think of at the start of the article. Oh come on it wasn't that long ago. That's the number of the lifeboat you are racing for.

OK, now unless there happen to be 20 of you gathered around the computer screen this is where you have to trust me.

Out of any random group of 20 people there is a very high probability that more than 5 will have chosen the number 3 lifeboat, so some of you will be in the water, or will go down with the ship. That means that even though SS Itil had sufficient capacity on paper it didn't in reality. On every single occasion we ran this exercise, and that is a lot of times, lifeboat number 3 was overloaded

This illustrates a very important point about capacity management. design a system to cope with an evenly distributed  average capacity and it will fail. Not "it might fail" but "it WILL fail" . Maths can prove the point*.

Why do so many people go for number 3?  That has less to do with maths and more to do with psychology. First of all people will tend to ignore  lifeboats 1 and 4 because they are the "obvious" choices.That leaves a choice between 2 and 3, Remember how I asked the question? " Think of a number between one to four." Without realizing  it people hear  "a number between 1,2,-,4" and fill in the missing gap in the sequence.

* Unless the underlying capacity requirement is absolutely even so the average= the maximum and the minimum


  1. So, it shows how important Demand Management really is? Lifeboat 3 should hold more passengers?

  2. Or you use psychology to alter the behavior of the passengers - or just ket the women and children go first....

  3. I think the interesting point here is the connection between Capacity Management and Demand Management. ITIL v3 has Demand Management as a sub-process of Capacity Management, but actually it deserves a much more significant role. You see, in the world of dynamic, on-demand, flexibly provisioned capacity (I didn't use the c-word!), managing demand is probably more important than managing capacity. Perhaps soft-controls like cost allocation (aka chargeback) might actually prevail in this environment?