As mentioned before, lots of data points are gathered and averages are often used to reduce this amount to smaller proportions. The ultimate goal is often to reduce different KPIs to one single number: the service level.
Traditionally, a service level is either defined as the result of one KPI or as the (weighted) average of a small set of KPIs, most often expressed as a percentage. This way, it is at best an average of a limited number of KPIs.
In practice, one often measures the service level and takes a baseline which is then used as benchmark.