Is there a formula for using metrics to calculate call center efficiency, either at a micro or macro level?
Many metrics reflect call center efficiency from different perspectives, and I don't think you can point to one particular way to calculate it. For example, utilization is one important measure of call center efficiency. Some people view average handle time (AHT) as a measure of efficiency for representatives, but fast handle time may not be in line with your other goals. Some would cite first call resolution (FCR) as a key measure of call center efficiency, while other centers know that the nature of their contacts or their inability to effectively measure FCR makes that a difficult metric. Some companies would use the self-service success rate as a key metric, while another company may not use self service.
The point of all these examples is that you need to define a metrics strategy that works for your business – tied to the goals you are trying to achieve, your call flow and handling, and the things you can measure. Efficiency or productivity should be one metrics category in a balanced strategy, but can be defined in various ways, depending on your environment.