In the last installment of our Operations series, we showed you how to create a process flow diagram, but we didn’t show you what to do with it. The usefulness of the diagram is that it makes it easy to see which groups of customers are placing demand on each resource. This information, combined with the inherent capacity of each resource, gives you the “implied utilization” of each resource, that is, the work that each resource would do relative to its capacity, if all the flow units moved unimpeded through the system. Whichever resource has the highest implied utilization is the bottleneck.

Start by calculating the capacity of each resource, by finding the inverse of the processing time and multiplying by the number of instances. So if each clerk on average processes a customer every 4 minutes, and there are two registers open, then the checkout capacity per hour is 60/4*2, or 30.

Next, calculate the total implied demand on each of the resources, by adding up the demand that would come from each flow path through the system. This is just a sum of the demand coming from each flow path, with no demand from any path that does not affect the resource in question.

Finally, find the implied utilization by dividing demand by process capacity (since utilization is just how much of the resource’s capacity will actually be used).

If this number is higher than one, it means the resource cannot actually handle the implied demand. Whichever resource has the highest implied utilization is the bottleneck. In this case, it is the Customer Service station.