Estimates as Sensors

Note: this is not an April Fool – honest!

I’ve been watching the #NoEstimates conversations with interest and decided its about time to pitch in with my own perspective. I don’t want to take any ‘side’ because like most things, the answer is not black or white. Estimates can be used for both good and evil. For me they are useful as a sensing mechanism. Put another way, by estimating, I can get a sense of how well I know my actual capability.

Lets take an example. I’m taking part in a 10K run this Sunday (*) and I am estimating that I will complete the distance in 55 minutes. This is based on an understanding of my capability from participating in regular 5K runs, and more general training runs over a 10K distance. I’ve never run an official 10K race, let alone this course, and I don’t know what the conditions will be like, but I’m aiming for 55 minutes. If I run quicker and do better than my estimate, then my actual 10K capability is better than I thought. If I run slower and do worse than my estimate, then my actual 10K capability is worse than I thought. Either way,  I will learn something about how well I know my 10K capability.

What helps even more with that learning is regular feedback! I use MayMyRun on my phone to track  progress and give me feedback every kilometre for total time, average pace and last split. This could be considered a distance-based cadence. I could probably also use a time-based cadence to give me equivalent feedback every few minutes. This feedback on a regular cadence helps me decided whether my estimate was way off, or if I should slow down because my pace is probably unsustainable, or speed up because I feel I can push harder.

How does this relate to product development? Well, we can use estimates in the same way to get a sense of a teams delivery capability, and use regular feedback to learn whether we’re making the progress we thought, and need to re-plan, slow down or speed up. Time-boxing, with Story Point estimates and Velocity can provide this time-based cadence and feedback. Alternatively, how long it takes to complete 10 User Stories can be used as a distance-based cadence and feedback.

To sum up, I recommend using estimates to sense capability and create feedback for yourself. I don’t recommend using them to make promises and give guarantees to others. Or maybe we could just call them sensors instead of estimates?

(*) Of course this post is primarily an excuse to invite readers to sponsor me. If you’re so inclined, or would like to show support for this post in a charitable and financial way, then please head over to my JustGiving page and do so there 🙂

Update: My final time was 49:23 based on the timing chip, or 49:30 from the starting gun. I’ve learned that I’m better than I thought I was, and next time I’ll be estimating 50 minutes!

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The Science of Kanban – Process

This is the third part of a write-up of a talk I gave at a number of conferences last year. The previous post was about the science of people.

Even though a kanban system describes knowledge work, we can still apply formal sciences such as mathematics. Rather than applying them at a detailed, micro level, we can take a system approach and apply them at the broader, macro level. As an example, if we try and predict the result of a single coin toss, we are only able to do so with 50% accuracy due to the unpredictable, random nature. However, if we to try to predict the result of 1000 coin tosses, we can be more accurate, albeit with less precision and some degree of variance.

Queuing Capacity Utilisation

The most common mathematics used with kanban systems is associated with queues and utilisation. Queuing capacity utilisation suggests that as capacity utilisation increases, queues increase exponentially. Thus extremely high utilisation will result in large queues.


Further, Little’s Law tells us that the time something is in a queue (lead time) is equal to the size of the queue divided by the processing rate.

Lead Time = WIP / Processing Rate

It follows then, that if we want to complete work quickly (i.e. with a short lead time), then should reduce queues, which requires reducing utilisation. Reducing utilisation too much, however, will begin to have diminishing returns.

Traffic Flow Theory

Traffic Flow Theory gives an explanation as to why a work-in-process limit on one may be too low.


Originally observed by Bruce Greenshields in 1934, Traffic Flow Theory describes how, as speed (in miles per hour) goes up, the density of traffic (in vehicles per mile) goes down, and vice versa. This is observable on roads with a low density of cars (i.e. empty) which are able to travel at high speeds, and roads with a high density of cars (i.e. gridlocked and bumper to bumper) which are stationary. Flow is defined as the product of speed and density, and thus there is an inverse u-curve, with high flow in the middle, and low flow at either end due to either low speed, or low density.

For product development, density can be viewed as work in process, speed correlates to lead time, and flow correlates to throughput. If work in process is too low for the available capacity, then lead time will be short, but throughput will be low. Similarly, if work in process is too high for the available capacity, then lead time will be long, and throughput will be low.

An interesting effect happens on either side of the flow curve.

On the left side, if density is increased (increasing work in process), speed decreases (increasing lead time), and flow decreases (decreasing throughput). The decreased flow causes a further increase in density resulting in a negative reinforcing loop.

However, on the right side, if density is increased (increasing work in process), speed decreases (increasing lead time), and flow now increases (increasing throughput). The increased flow causes a decrease in density resulting in a balancing loop.

Thus, while the optimum density is unlikely to be achieved precisely, it is better to be on the right hand side, with slightly less work in process, where a slight increase will balance itself out.


It’s generally recognised that software development is a non-linear activity, due to the many feedback loops, so it’s useful to understand the effect that the feedback loops have on a process.

One of the impacts that a feedback loop has is related to its delay, where a long delay can lead to instability and oscillation. Taking a shower as an example, if when trying to turn the heat of the shower up, there is a long delay between turning the tap and the water temperature increasing, the natural reaction is to turn the temperature tap further. Eventually, when the change in water temperature comes through it will be too hot. The same can happen again when trying to turn the heat of the shower down again.


Thus it is a good idea to minimise delays in feedback, or put another way, have fast feedback. Fast feedback also results in smaller queues, because less work builds up waiting for feedback. Smaller queues result in less work in process, and we have seen that lower work in process leads to shorter lead times. Shorter lead times further generate faster feedback, resulting in a positive reinforcing loop.


In the next part we’ll cover the science of economics.

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