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Erlang Library for Excel Function Reference |
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Erlang Library for Excel Function Reference |
Returns the number of servers needed to carry a specified traffic load with a specified service level.
ErlcNsrvFromFractionOk( trafficInErlangs, ahtSeconds, okWaitSeconds, fractionOk)
trafficInErlangs is the offered traffic load in erlangs.
ahtSeconds is the average handle time (average duration of service).
okWaitSeconds is the maximum acceptable wait time for customers.
fractionOk is the fraction of customers who begin service after a wait that does not exceed okWaitSeconds.
| Formula | Description | Return Value |
|---|---|---|
| =ErlcNsrvFromFractionOk( 10.3, 180, 10, 0.90) | Returns the number of servers required to carry 10.3 erlangs of traffic, with average handle time = 180 seconds, okWaitSeconds = 10 seconds, and fractionOk = 0.90 (that is, 90%). | 14.9274 |
| =ErlcNsrvFromFractionOk( 10.3, 180, 10, 0.95) | Same as above, except fractionOk = 0.95 (or, 95%). | 16.0653 |
Public Function ErlcNsrvFromFractionOk(trafficInErlangs
As Double, ahtSeconds As Double, okWaitSeconds As Double, fractionOk As Double)
as Double ErlcFractionOk, ErlcNsrvFromFractionOk5, ErlcTrafFromFractionOk
This is one of the most popular of the Erlang functions. You use ErlcNsrvFromFractionOk to estimate number of agents required to staff a call center (see the Case Study below).
In call center applications, it is often more convenient to use the closely related function ErlcNsrvFromFractionOk5.
If this function is not available, and returns the #NAME? error, then you must install and load the Erlang Library for Excel from Abstract Micro Systems.
How?
Suppose that during a certain period of the day, a call center has these characteristics:
| Maximum number of calls that can be waiting: | Very large |
| Period length: | 30 minutes |
| Incoming calls offered during the period: | 1234 |
| Average handle time: | 323 seconds |
| Service Level Goal | 90% answered within 20 seconds |
How many agents are needed to achieve the service level goal?
Suppose further that calls arrive in a Poisson process, that service times are exponential, and that callers who must wait are infinitely patient and will stay on the line until their call is answered (i.e., that no callers abandon). Then the call center satisfies the assumptions of the Erlang C queueing model, so we can make a staffing calculation using ErlcNsrvFromFractionOk.
First, we must compute the offered traffic x in erlangs. We find:
x = (calls per period) * (average
handle time) / (period length)
= 1234 * 323 / 1800
= 221.434 erlangs
Therefore, the required number n of agents to achieve the stated service level goal is
n = ErlcNsrvFromFractionOk( 221.434,
323, 20, 0.90)
= 235.88
We conclude that we need about 236 agents to achieve the desired quality of service.