The single-file line analogy identifies the concept of bottlenecks.

Bottlenecks are the slowest part of your system: the slow kid in line, the slowest machine, the narrow part of the clear plastic container through which liquid flows.

Bottlenecks determine your throughput.

When you have a number of processes required to ship a product (or complete any task), you can’t possibly go faster than your bottleneck will allow. A series of processes is like a chain – it’s only as strong as the weakest link.

You can work to make bottlenecks have more throughput – and we’ll get to that – but step one is identifying them. You must identify bottlenecks in order to manage a system with flow. Why? I just explained it – bottlenecks dictate throughput.

When Alex put the slow kid at the front of his line, he identified the bottleneck and used it to set the pace. Putting fast kids in front of the bottleneck/slowpoke serves no purpose; they might get to the campsite early, but that’s not “throughput” – the entire troop arriving is throughput. The fast kids are most useful behind the bottleneck, where they can catch up if and when they the fall behind due to random fluctuations like stopping to tie their shoes.

This analogy definitely helped illustrate the concept of what happens in a factory. When a process is a bottleneck, inventory can stack up behind it and idle time can be created in front of it. That causes all kinds of issues with inefficiencies and inventory, both of which drive up cost.

But before I go on to describe that stuff in more detail, one more analogy from the book in the next post.