Standard call center equipment and staffing decisions rely heavily on traffic engineering calculations. The average speed of answer -- or ASA -- uses the Erlang-C probability formula to calculate the average amount of time a customer waits on hold before a call center agent answers the call. Although you most often calculate the ASA using specialized software or a spreadsheet macro, understanding and being able to calculate the bulk of the process manually -- with a bit of assistance from a graphing calculator -- can help you fully appreciate what goes into and ultimately affects ASA.

Calculate the average call arrival rate by dividing the number of calls you receive in 30 minutes by 30 -- the number of minutes -- or by 1800 -- the number of seconds in 30 minutes. For example, 400 calls / 1800 seconds = 0.22 calls per second.

Calculate the average length of each call in a 30 minute timeframe. Using an external call report, add the total minutes -- or seconds if you use seconds in the first calculation -- and divide by the number of calls. If 120 calls equal 965 minutes or 57,900 seconds, average call time is 482.4 seconds.

Determine traffic load, also called traffic intensity, by multiplying the average call rate by the average length of each call. For example, multiply 0.22 calls per second by an average call time of 482.4 seconds per call to get a traffic load of 10.61.

Get the number of agents available for taking calls within the same 30-minute timeframe and divide this number by traffic load to calculate an agency utilization rate. If you have 90 agents available for taking calls and a traffic load of 10.61, your utilization rate is .848, or 85 percent.

Calculate the probability a customer will go to a hold status rather than speak to an agent immediately. Enter the numbers in the previous steps -- where “u” is traffic intensity, “m” is the number of available agents and “p” is the agency utilization rate -- into an Erlang-C probability equation using the formula “Ec(m,u) = um/m! um/m! + (1 − ρ). ∑m−1 k=0 uk/k!” and a graphing calculator.

Calculate ASA by first multiplying Ec(m,u) by the average length of each call and then dividing the result by the number of available agents x 1- the agency utilization rate. If, for example, you get an Ec(m,u) of 0.189, the calculation reads as 0.189 x 482.4 / 90 x (1-0.85) or 91.17/13.5. The ASA at this time is 6.75 seconds.

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