# Monday, 22 September 2014


Numbers Game

Whenever I’m in a meeting talking about metrics, someone always brings up Velocity. If you’re familiar with Scrum you’ll know that in this context, velocity means the number of story points completed per sprint. If a team completes 4 stories each of which scores 5 story points in one sprint, the velocity of that team is said to be 20.  If that doesn’t make sense, you should probably do some googling about now and come back when it does… don’t worry, I’m happy to wait for you. Look up planning poker while you’re on. ;-)

Ok ready to continue? 

Velocity isn’t a metric

No, Velocity can never be a real metric, and to use it as such is to play a dangerous game. But let me explain why this is so, and if I’m taking it away, what you can replace it with (or use alongside) as a real metric. 

Numbers are Magic

It is my belief - although as yet I do not have any scientific data to back myself up - that numbers hold a special place in our minds. Numbers and Mathematics are after all an abstract construct developed by humans as a language to explain science. It is my premise and belief that we see things that are expressed as numbers as things that we can control. If my mass is 125 Kilograms, I can target loosing 5 kilograms then go on a diet and exercise regime, and measure my progress with my measured mass on any day. 

When is a number not a number?

What madness is this? Well I’m trying to show the difference between a real mathematical number like say 3.142 and a string of digits like 055555 555 555. You probably recognise both of those, one is a shortened for of Pi, which we use in trigonometry to work out areas and circumferences of circles, and the other is a fake uk phone number. However doing maths makes sense with Pi, it doesn’t with a phone number. Imagine adding 44 to them.

x= 44+ 3.142, so x= 47.142

y= 44 + 05555555555  ….

While you can work out that y = 5555555599, it really makes no sense, what we really want to do is manipulate the string of digits to be  +445555 555 555 - the international form of the phone number. 

So some numbers are real mathematical numbers and others like phone numbers are just strings of digits, so we need to very careful whenever we use something that looks like a number but is really a string of digits as people will most likely not understand the difference without explanation and try to use the number as a real mathematical number and a control point. 

Velocity is a tricky one as it looks like it should be a real number, after all we can “Measure" it for a team. But appearances can be deceptive. My old Physics teacher at secondary school once told me that “if the maths is ever confusing, look at the units and they will help you understand what is going on.” Wise words for maths, physics and velocity. Velocity is measured in “story points completed per sprint” which might as well be “tulips completeted per sprint”  The problem with that is that it is a malleable number. 

Malleable Numbers?

Malleable Number

What do I mean by a malleable number? Malleable means “easily influenced; pliable”. So let’s pretend I’m a Command and Control manager who is up against it for a project. I see Team A with a velocity of 22, and Team B with a velocity of 35. If I think these are real numbers and in my mind they are therefore control points, so I would be likely to ask the question, "how come Team B is so much better than Team A, and why can’t Team A do 35 points per sprint too?”

We, as the kind of people who read blogs like this, probably know innately that that question is a danger sign, but lets follow it through.

Team A get pushed into working towards a target velocity of 35 points. So what are they likely to do in the next story grooming (refinement) meeting when they come to estimate their stories? I suggest that a story that would have been a 5 last time round will now be an 8 - or even a 13 point story. Why am I so confident? I’ve seen it happen in my own teams. But that is the least of the problems.

As a free gift with the instruction from manager to team there is a side order of “understanding that Management don’t seem to have a clue” which undermines the organisation, and pushes the team towards being more insular and deceitful.

It has a negative effect for everyone. Even if the team doesn’t “cheat” on estimation, they may well start “sand bagging” the sprint by working longer or extra hours. And of course that will artificially uplift velocity at first, but as people get tired, the velocity will actually drop (again I’ve seen that in action) as people stop working in a sustainable manner. 

As soon as the thing we are trying to control is malleable, as human animals we can’t resist playing with it. A bit like a blob of blu-tac on your desk, it’s malleable and you can’t help but play with it. How do we solve that problem? We put the blu-tac away in a packet, drawer or cupboard. We need to keep the malleable stuff away from the people who want to play with it - so it is with velocity. Just like blu-tac, velocity is a useful tool when used correctly (like for deciding when to stop grooming stories at the story grooming meeting), but should be put out of reach the rest of the time. Use something which isn’t malleable as your metric - like how many days stories take to deliver. In the Kanban world we call that the Lead Time, and it is measured in days (or sometimes hours), a fixed unit of time. Unless we solve the problem of light speed travel, fixed units of time are Immutable Numbers.

Tags: Agile | Estimation | Kanban | Lead Time | noestimates

Monday, 22 September 2014 13:53:25 (GMT Daylight Time, UTC+01:00)  #