Why you should limit work in progress and stop multitasking

Limiting work in progress and stop multitasking is the best advice you can give to any team or individuals. This is an illustrated attempt to convince you to limit your work in progress and stop multitasking.

Suppose your job is to build houses: each step takes one day, so you need 5 days to deliver one house to your client.

In lean terminology : your cycle time is 5 days (the average time a task stays in your system).

But if you work on four houses at the same time (increasing your work in progress), your cycle time will almost quadruple : on average your customer will have to wait 18,5 days.

The more you add work in progress in your system the longer will be your cycle time. Your average delivery rate will increase linearly for each concurrent project you add to your workload.

Queuing theory is the mathematics of waiting lines. Little’s Law states that your cycle time (time waiting in the queue) will increase with your work in progress (task or people already in the queue).

To deliver faster you have two options: limit your work in progress or increase your throughput (hire more people, make people work overtime…). Guess which option is the cheaper ?

By limiting work in progress you deliver faster by doing less at a time.

Stop starting ! Start Finishing !

More queuing theory : the Kingman’s Formula states two things:

The higher your utilization, the longer your queue. Eventually your queue will approach infinity as your utilization approaches 100%.

The second factor is the variation. The higher your variation, the longer your queue.

Those two factors are not added but multiplied with each other. So, while a high value in each is not good, a combination is even worse.

Again, guess what is (generally) easier between reducing variation and limiting work in progress ?

Limiting work in progress is often far easier than trying to reduce variation !

While this seems simple and intuitive, most managers are not inclined in minimizing utilization : there’s work to do, everyone should be busy!

You must be careful: you get what you optimize for !

If you optimize for busy people you get … busy people!

By limiting work in progress you optimize for flow (lower cycle time) : you introduce slack in your system !

To preserve flow you need some slack: an interstate at 100% utilization is a parking lot !

By limiting work in progress you will decrease context switching.

Our brains are not designed to focus on multiple tasks at a time: when you think you’re multitasking you’re only switching fast between two tasks.

Every interruption or task switching has a cognitive cost.

The cost of bergswitching is often underestimated : In Quality Software Management, G.Weinberg found that every project added to the workload results in a 20% drop in productivity. If you add one more project ? Another 20% drop in productivity !

With five simultaneous projects only 5% of the work time is available for each project ! Context switching is a huge waste of time and money.

Multitasking also lowers your work quality. A study at the University of London showed that subjects who multitasked while performing cognitive tasks experienced significant IQ drops, similar to what you see in individuals who skip a night of sleep or smoke marijuana.

Finally, multitasking is bad for your health : multitasking increases production of cortisol, the stress hormone.

Here’s the complete illustration :

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