Service level metrics

We saw that the goal of call center management is to obtain the right cost-service level
trade-off. We also saw by who and by what types of decisions cost and service level can
be influenced. We now go into more detail how this service level can be defined.
The service level (SL) obtained by a call consists of several different aspects. Several
of these aspects are related to the handling of the calls themselves, such as the way in
which the agents attend to the call, and the ratio of calls that need no need further calls,
the first-time-fixed ratio. Others are related to the waiting process, notably the waiting
times and the occurrence of abandonment. We focus on waiting times and abandonments,
although other aspects of the service level can have a large impact on the waiting time and
therefore also on the abandonments.
The help desk of an Internet Service Provider had a considerable rate of callers that phoned
back after their call because the answer was not sufficiently clear to solve their problems.
By improving scripts and documentation and by additional training this rate was reduced
considerable. This not only improved the perceived service level, it also reduced the number
of calls. This had a positive effect on the waiting times, and thus again on the service level.
The common way to define service level is by looking at the fraction of calls that
exceeds a certain waiting time, which we will call the ”acceptable waiting time” (AWT).
The ”industry standard” is that 80% of all calls should be answered in 20 seconds, but other
numbers are possible as well. The SL can simply be calculated by dividing the number of
calls handled before the AWT by the total number of calls.
Often we know the SL for short intervals (often giving by the ACD), and we want
to compute the SL for longer intervals, for example in a spreadsheet to make a monthly
report. The SL of a long period composed of several shorter of which we know the SL can
be calculated be averaging in the right way service levels over the shorter periods. When
averaging over a number of intervals the number of calls in these intervals should be taken
into account. Consider the table below. At first sight the average service level is 75%, by
averaging the four percentages, but now the differences in numbers of calls per week are not
taken into account. The right way of calculating is to compute the fraction of calls in each
interval first. For example, the fraction of calls in the first interval is 2000
17000 , 17000 being
the total number of calls over the four weeks. Using these fractions a weighted average is
calculated in the following way:
200017000× 95 +700017000× 55 +500017000× 70 +300017000× 80% = 68.5%.
Week Number of calls Answered within 20 s. SL
1 2000 1900 95%
2 7000 3850 55%
3 5000 3500 70%
4 3000 2400 80%
This way of calculating averages corresponds to the answer in case the service level was
computed directly for the whole month. Indeed, out of a total of 17000 calls 11650 were
answered in time, thus a 11650
17000 × 100 = 68.5% service level.
The difference between 68.5 and 75% is not that dramatic. This is because the number
of calls in the different weeks are roughly of the same order of magnitude. If the number of
Chapter 3 — On call center management and its goals 11
calls in the intervals over which we average are very different, then the way of averaging can
have an even bigger impact on the result. These big fluctuations typically occur during
days. At peak hours we can easily have ten or twenty times as many calls per hour as
during the night. Then the difference between ways of averaging can run into the tens of
percents.
The percentage of calls that is answered in less than a certain fixed waiting time is
sometimes called the telephone service factor (TSF). Another commonly used waiting time
metric is the average speed of answer (ASA).