I have seen a number of discussions recently about velocity, and generally the harm it can cause when used incorrectly. Most seem to advocate not using it to avoid mis-using it but whilst it is a blunt instrument it is just information like any other metric and for a process that relies on empirical evidence to adapt and evolve it seems odd to ignore a very useful metric, so instead I will try to explain how, when and why velocity could be used to add value.

My car has a trip computer and among its varied metrics it offers an ‘instant fuel economy’ feature.  Why? I don’t know, but to my mind it is a pretty useless feature, but it is there and at any time I can take a spot check and it will tell me how much fuel I am using in mpg.   When racing up the hill to the Air Balloon roundabout foot down it drops to around 20 mpg, when coasting down with my foot off the pedal I get 99 mpg.

If I were to take a single reading half way up the hill and claim that my car does 20 mpg I would get a totally distorted view of the velocity with which I consume fuel, equally if I took a single reading coming down at 99 mpg.    I’m sure most of you would think I was nuts to even consider taking a single reading as an absolute indicator of future velocity. But somehow with ‘sprint velocity’ people think that is okay.

Let me stretch the analogy further. I know that a single measurement is a little crazy to base things on, so let’s take an average over the last 100 miles. That is a pretty solid and reliable metric, most of us would consider that to be a reasonable predictor.   But I have spent the last few weeks only driving around town, stop start, traffic jams, short journeys and so on. But now I have a long trip, mostly motorway, will my average fuel consumption velocity be a good indicator of the next 100 miles?  No it won’t, common sense tells me before I even look that my velocity will be different under different conditions.  A velocity only has meaning if the measured data is representative of the future conditions.  Equally I tend to drive with a heavy foot, but I am quite sure if my father were to drive the same journey in the same car the consumption velocity would be quite different.  The same is true if I drove a different car, the velocity would be different even if my driving conditions were the same.

Velocity tells you only one thing – what was consumed in this car in these conditions. It offers no view of the future, or of alternative conditions.

But I am free to interpret, I can predict, I can guess, I can extrapolate all I like, but it is not the velocity that comes up with these figures – it is me. If I make a prediction and it is wrong, I cannot blame the velocity, the velocity is a measurement of what was, not of what will be.

So how is it useful then?    The most obvious is to say that if I believe that my typical journey over the next 100 miles will be similar to the last 100 miles, similar driver, similar journeys, similar car then it is not unreasonable to expect similar results.  It can be a predictor.

Or let’s say that I am a little bit eccentric and I decide that I want to increase my average MPG and I spend the next 100 miles on similar journeys in a similar car, but I try very hard to improve my driving to be more efficient, in that case it can be a blunt measure of improvement in my driving if the average goes up, I can measure improvement.  I would caution myself very carefully on this particular measurement because it could easily be a change in the road conditions not the driver that is the cause for a change but even so it is a measure – the interpretation of meaning is mine.

Or if nothing changes that I am aware of and the average goes noticeably down or up, it might be an indication of a problem it might be an indicator there is a problem with the car, or my driving, or even just different fuel.

So now I understand all about the velocity of fuel consumption my current average is 50mpg. How much fuel will I consume on my next journey?     There is no way to tell, to even make a reasonable estimate I’d need to know an approximate distance, I’d need to know the terrain and even with all that there are still unknowns, I may hit a delay in traffic, or a diversion, I may get asked to call in and pick up milk, I may even breakdown.  And if I could predict all of that and gave you an estimate, would you trust it to any degree of accuracy?  I wouldn’t, I might trust the estimate to a degree, but I wouldn’t play to fill up on empty on such an estimate I’d almost certainly plan a significant buffer to ensure I didn’t run out of fuel.

I hope most of you would agree with my views on fuel economy, there are simply too many unknowns to make accurate predictions.

And yet, I reset my trip computer fairly regularly every 1000+ miles or so, and on average over that 1000 miles my economy is in the region of 48 ish, it very rarely fluctuates significantly from this. Over a very long timeframe I can get a very accurate estimate, that is consistent 1000 miles after 1000 miles.  That still tells me nothing about the next 20 miles or even 100 but long term I can be consistent.

Another interesting observation is that to get this measurement I am using an extremely accurate tool to measure, I am in a known unvarying state – same car, same driver, similar journeys.  And yet with all this certainty I still don’t trust my predictions to any degree of accuracy I buffer I plan contingency.   But if I stop talking about cars for a moment and I talk about software, more specifically a team developing complex software with a great many unknowns, where the team members can take leave, be sick, have family troubles, they can leave the team, or new people join the team, they can learn new skills, use new tools.  But we are expected to estimate, often to a very specific dates, large quantities of unknown and often very complex work. What is more odd is that those estimates are often taken as commitments.

If a rational person wouldn’t trust a reliable and regular machine like a car to give specific accurate predictions with fairly well known parameters, why do similar rational people expect such accuracy from a domain far more variable, far more complex and far more unpredictable?

My rather long winded discussion on fuel economy is really summed up in one sentence.

There is only one estimate on how long a software project will take that I would trust and that is one given the day after the project is completed.

Otherwise use projection tools like velocity in context,  if the future conditions are likely to be unchanged then velocity can be a useful tool, but the longer the average the better.  And if your circumstances change: the team, the type of work the domain, the tools your current velocity is meaningless until you have enough information to create a new measurement.  And think of your car, what value is an snapshot measurement without context.

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