Quantifying the effect of a service outage

A method is described for quantifying the effect of an outage in a service over a first period of time. The method comprises measuring usage of the service over time 100, defining a cost of outage time window 106 comprising the first time period and a second time period following the first time period, and computing a cost of outage as the difference between the measured service usage during the cost of outage time window with service usage measured during a comparison window 118. The comparison window is substantially equal in time to that of the cost of outage time window and reflects a similar period of service activity as that of the cost of outage time window without having a service outage. The onset of a service outage may be determined by monitoring a drop in service usage or service quality and the second period may be related to the time which it takes the service to return to a given percentage of normal service usage. A third time period may be used to determine the long term effect of the outage after the return to the given percentage of normal service usage. Comparison of the cost of service outages for different services enables prioritisation of outages based upon the compared costs. Also mentioned is a method of predicting causes of network service outages using time domain correlation.