Remy,
First, I am not sure I understand your statement AHT for the big Callcenter is aimed at 250 seconds en the AHT for the small Callcenter is aimed at 550 seconds AHT is a property of the call. You can strive to improve it but you cant control it directly. For the purpose of this discussion we will assume AHT of 200 seconds in both call centers.
More importantly, I ran your numbers (AHT=200, target SL=70/20) on my software and verified the results using other Erlang calculators. The results are different:
CPH --- Agents --- ASA ---- Occupancy
10 --- 2 --- 17 ---- 28%
100 --- 8 --- 21 ---- 69%
1000 --- 60 --- 20 ---- 93%
As you can see, the ASA is pretty constant, but the utilization increases. (The reason for the more pronounced difference in ASA in the very low call load is that agents (and telephone trunks) are integers, so in a very small call center adding/removing an agent causes a significant change.)
Did you use a commercial Erlang C tool for your calculations or was it developed in-house? For example, your formula for occupancy doesnt seem right. What is workload in your definition workload/ number of agents? Occupancy rate is net traffic/number of agents. Using my calculations above for 100 CPH occupancy rate for 8 agents is 100/18/8=69%.
Using your number of agents (26) you get 100/18/26=21% which is inconsistent with the 85% you have calculated.
You may want to try another tools (e.g.
www.DiagnosticStrategies.com/EasyErlang.htm) to check the numbers and verify that you are still seeing the (significant) discrepancy in ASA.
Interesting discussion!
Joe