You gently tilt a bottle of wine (or soda, if you’re a teetotaler) and watch a steady, slender stream of your liquid of choice descend into your glass. That’s nice, but you want more, faster, so you turn the bottle up a little more. The stream widens (and so do your eager eyes). But it’s still not enough, so you turn the bottle on its end. Suddenly that steady stream disappears and is replaced by a bursty, messy, series of explosions. Somehow, less liquid comes out of the bottle instead of more, and half of what does come out lands on the table instead of your glass.
Okay, you get it: it’s a metaphor. But what does it mean? Let’s start with a little terminology and all will become clear. Most processes can be divided up into groups of tasks or activities, according to which resource has to be committed to carry them out. The processing time for each of these resources is the length of time it has to be committed. The capacity is just the inverse of the processing time: the number of flow units (defined here) that can be processed by the resource in a unit of time.
Now you can probably guess that the bottleneck in any process is just the part of the system with the lowest capacity (and the longest processing time). Of course, the process capacity, or the amount of flow units that can emerge from the whole system in one unit of time, is going to be identical to the capacity of the bottleneck. A system operating “at capacity” is releasing exactly that number of flow units in any given unit of time.
But wait a sec. In our metaphor, the flow of liquid out of the bottle was not the same at every moment. It came out in unpredictable, chaotic bursts. What’s happening physically is that the standing inventory (wine/soda) behind the bottleneck is interacting with the bottleneck in an undesirable way. Ideally, standing inventory just waits, and then flows again after the inventory in front of it has been processed. But liquid compressed between gravity and air pressure waits, as we might say, antagonistically.
If your flow units are customers, you can bet that this kind of antagonistic waiting is going to be an issue. Customers standing in line at checkout or waiting to be seated aren’t going to wait forever without complaining. It’s not just about the numbers. Like a deft sommelier, you need to keep the flow working with you by controlling the contrary pressures. Think about how can you make waiting a positive, or at least a neutral experience for your customers. I’ll be back next Thursday with more ideas on this topic.