# WiP Limits and Velocity

I recently presented at a conference on the topic of Work in Progress (WiP) limits, and in particular Little’s law.  As part of I gave a demonstration and from it drew the conclusion that WiP limits have no direct bearing on velocity.

This caused some controversy so I’ll try to explain again here.

### Applying Little’s Law

Mathematically speaking we can increase the speed with which we get served in two ways:

1. We reduce the number of people in the queue (We limit WiP)  or
2. We increase the speed each person is served (We increase Throughput – in this case by adding more people, or installing faster equipment)

Note:  When mathematically applying Little’s law the pre-condition is that the variables are independent, the assumption is that number of people in the queue does not impact on how quickly the server works.  This works in mathematics, but maybe not so much with people in reality.

The principle is that WiP and Throughput are two independent variables and changing one does not change the other, only cycle time is impacted.

### The wider impact of WiP limits

Of course the goal of my talk was to illustrate that whilst you may feel you are getting more done by not limiting your WiP, and having a whole bunch of work in progress this has no bearing on your actual throughput which is governed by your bottle neck.

When using Little’s Law in a controlled environment we can show that limiting WiP doesn’t impact throughput (unless we limit it to less then our bottleneck and starve it – say two servers and limiting the queue to 1 person). What limiting WiP does do is reduce cycle-time enabling you to be more flexible and more responsive and to have more reliable forecasts.

### Work in Progress Limits will directly impact Cycle-time

Unfortunately this contradicted one of my other claims in the presentation where I said that Limiting WiP helps increase throughput of the team.

The confusion of course is understandable, when using a mathematical example there is no cost for context switching and the example was to show the impact of WiP on a stable system isolating variables to demonstrate what happens.  The real world is far more complex and there are many more variables.

### In the real world

Generally speaking the ability to focus on less things means less multi-tasking, reduced cycle time means you have less things on the go at once, and for less time. These help you be more productive and thus indirectly the throughput will go up, but this is a result of you being able to focus not as a direct impact of the WiP limits.

There are many indirect benefits of using WiP limits,  but Cycle-time is the prime beneficiary, the rest is a bonus or an indirect consequence.

# Why do we use WiP limits?

I participated in a great discussion this week on the use of WiP limits.  For those that don’t know, WiP stands for Work in Progress, and is a measure of how many activities you have started but not completed.

It is important to note that this is not a measure of how many tasks you are currently actively working on.  If I started a task but have become blocked, so I start another then my Work in Progress is 2 – even though I am only actively working on one. WiP is a measure of how many started but incomplete tasks I have.

For example: call-waiting or being put on hold during a phone call:  You are talking away and get another call so you switch to the second call without hanging up, and when you get another call you take that too, how many people can you have on hold at once?  You are only working on one call at a time, although you must remember the content and context of all those other calls. For the people still on hold while you have all these conversations I expect they are wishing you had a WiP limit.  Your WiP-Work In Progress is the sum of all active(incomplete) phone calls.

## Limiting WiP

There are many reasons for limiting Work in Progress not least because I hate being put on hold.  I will go in to some later and it is likely that if you follow any type of Agile framework you already limit WiP although you may not be consciously aware of it.

If you have a backlog, then you are limiting WiP. You are consciously choosing not to start new work immediately but are prioritising it and organising it to work on later. It is likely that you are informally limiting your WiP according to what you feel you or your team can manage at a given time. It is important to realise that this is a WiP limit even though it may not be conscious or scientific.

If you follow Scrum: WiP is consciously and explicitly limited at Sprint Planning, often by estimating effort, or counting stories or calculating story points. The limit is generally set based on Velocity – our average achieved over previous sprints. If on average we finish 10 stories a Sprint, or we average 35 points, then that average is generally taken as the starting point for our WiP limit, we may choose to push for a few more, or if we are expecting to be slower due to vacation time, or known issues we may choose to commit to less. But in Scrum we don’t usually refer to it as a WiP limit. But that is exactly what it is.

KanBan is actually very similar, if we on average are completing 10 stories a week, then it is likely that we will consciously or subconsciously limit ourselves to only take on 10 new stories within a week, although in the case of KanBan this is not done in a Big Bang explicit decision but gradually over the measured period and is more a consequence than a plan (A Pull rather than a Push). We pull stories when we are ready and we pull at the pace we are working at, that just happens to be 10.

Slightly off-topic, but one of the crucial differences between Scrum and KanBan is that Scrum is a ‘Push’ model and KanBan is a ‘Pull’ model.  In Scrum we Push an amount of work to the board and spend the Sprint working to Remove(complete) it.  With KanBan we are aiming for a more steady amount of Work in Progress and will pull new work as current work is completed, which creates a smoother flow, but conversely lacks a unified time based goal or target.  But it is important to understand that both frameworks limit WiP and both focus on ‘completed’ work being the primary objective.

## Why do we limit WiP?

I’d like to think it was obvious by now and I think the basic principle is, but there are nuances that may complicate things which may be where the cause for debate comes from.

At it’s simplistic level if I can only complete an average of 10 stories a week/sprint then starting an average of 20 a week without changing anything else is nuts, all that will happen is that on average 10 or more of those stories each week will remain incomplete and stay ‘in progress’ by the start of week 4 we will still have completed 30, but will have 50 now in progress.  Chances are by now we will actually go slower because we will be distracted and falling over ourselves trying to juggle more than we can possibly achieve.

Think of all those people on hold, growing frustrated at being ignored, and you trying to remember all those conversations, the phone rings again, can you cope with another caller on hold?

We limit WiP because we understand that by working only on what we can achieve, we can focus and be more effective.

## Limiting WiP and WiP limits

“Ah but you have talked about Limiting WiP but you haven’t mentioned WiP limits” I hear you say…

And this is where I think the conversation really stems from and where we get into nuances.  KanBan began in manufacturing where work was passed from one workstation to another and there was a measurable flow through the system.   Limiting the production on one workstation so that it maintained pace with the entire system limits waste. Having one hyper-performing efficient widget maker that can produce widgets 4 times faster than they can possibly be used is wasted effort. And the same applies to a software process, we use WiP limits to regulate one part of the flow to maintain pace with the flow as a whole, the goal is to visualise bottlenecks and by visualising bottlenecks we can take steps to increase the flow of the whole system.

## Other forms of WiP limit

But WiP limits can take many forms and are applicable to almost all aspects of life. The most common example used is a highway, when a road gets busy it slows down, when it gets very busy it grinds to a halt.  Limiting access actually speeds things up for everyone on the road, and ultimately more cars can get through far quicker.

But there are other less obvious WiP limits. A long-distance runner wants to increase her times, she starts fast but she runs out of energy and is slowing down near the end of the distance. By setting herself a slower time per mile early on (Pace is a form of WiP limit) she has a consistent speed and reserves energy so is able to run the full distance faster by slowing down during the early part of the race.

Are you dedicated to a single product/project?  That is a very low WiP limit of 1 project.

Do you use a calendar and try not to book two meetings at the same time, that is a very tight 1 item at a time WiP limit.

What about a budget?  Do you manage your finances as an individual or company. A budget is a WiP limit for your money, by limiting money spent on beer you have more available for clothes. By regulating your spending you can save for a holiday.  By remaining in credit at the bank you do not have to pay interest and so on.

## Example of how to choose a WiP limit in a workflow

In the case of the widget maker, if the system can only use 10 widgets a day then we may set his WiP limit to 10, when he gets to 10 he is free to go and help someone else.  Suddenly rather than unused widgets we have an extra pair of hands to do other work.   But hold on! these widgets are easy to make and only take a short while to do, we could probably limit WiP to a lower limit and jump on the workstation as an when needed to produce more.  So how do we set the WiP limit?  My advice is not very scientific at this point.  Start by continue doing what you currently do and just watch, if your workstation/activity columns are sub divided in to doing and done, take a look on a regular basis and see where the biggest stacks are. If one column regularly has more cards on the done column than doing, it ‘may’ be a sign that you should reduce your WiP limit for that activity. Try it and see if it improves your flow.

The largest queues of done work are usually the area that needs limiting, but be aware of ‘bursts’ of activity, for example there may be a workstation that is only available for one day a week, in which case the queue for that workstation may need to be longer – or maybe you should be asking for more regular access to the workstation. But when imposing limits do it gradually and monitor to see if it improves the flow.

Anything in a done column is normally waste, if it is done and not in production that effort cost you money and is just sat there.  Sometimes that makes sense but the car industry learned that complete cars sat in storage amounted to \$millions of wasted investment, that components stockpiled in advance was essentially big piles of cash that could be used more effectively, we can learn the same.

What I am saying is that a WiP limit being reached is a conversation trigger, a long queue is a conversation trigger, and so on. With so many things in Agile we create early warning systems to create conversations and make decisions when it is necessary.  We make small changes to the process and watch, we see what changes and if it is better we carry on, but look for another new small improvement.

## Summary

WiP limits are not unique to KanBan, and like it or not you are already limiting your work and activities in all aspects of your daily life, you just perhaps didn’t realize it.  In the context of a KanBan board we do not arbitrarily impose a WiP limit because we can or because it is fun. We impose a limit when we feel it adds value, normally where we see a queue of completed work growing faster than it is being consumed. We limit various stages of the work because our goal is the output of the system (the whole team) and not the perceived utilization of individuals.  One team member being 100% occupied all day producing something that will sit untouched for a week is not efficient.

We are not trying to manage a system where our goal is to keep busy a group of individuals, our goal is to create a cooperative and coordinated team working towards a common purpose in the most effective way possible. A WiP limit is just one of many tools that can be used to enhance the prioritisation of work and focus the team on the true goal.