A method of determining a measure indicative of expected quality of experience in respect of data traversing a particular end-to-end path across a network, the particular end-to-end path comprising a plurality of communication links which together form a path via which data may traverse the network from a data-sending network-node to a data-receiving network-node, the method comprising, in respect of each of a plurality of links of which said particular end-to-end path is comprised, determining a link performance measure by: receiving end-to-end path performance measures in respect of a plurality of monitored end-to-end paths across the network, the monitored end-to-end paths each comprising a plurality of links which together form a path for data to traverse the network from a data-sending network-node to a data-receiving network-node, the respective end-to-end path performance measures in respect of each monitored end-to-end path being dependent on and indicative of a network performance metric observed at the data-sending network node of that end-to-end path or at the data-receiving network-node of that end-to-end path; and determining, in dependence on end-to-end path performance measures received in respect of a plurality of monitored end-to-end paths having at least one link in common, and on the basis of route information identifying the links of which those end-to-end paths are comprised, one or more link performance measures the or each link performance measure relating to a link of which at least one of those end-to-end paths is comprised; then: determining a measure indicative of expected quality of experience in respect of data traversing said particular end-to-end path in dependence on the respective link performance measures determined in respect of the plurality of monitored end-to-end paths. receiving end-to-end path performance measures in respect of a plurality of monitored end-to-end paths across the network, the monitored end-to-end paths each comprising a plurality of links which together form a path for data to traverse the network from a data-sending network-node to a data-receiving network-node, the respective end-to-end path performance measures in respect of each monitored end-to-end path being dependent on and indicative of a network performance metric observed at the data-sending network node of that end-to-end path or at the data-receiving network-node of that end-to-end path; and determining, in dependence on end-to-end path performance measures received in respect of a plurality of monitored end-to-end paths having at least one link in common, and on the basis of route information identifying the links of which those end-to-end paths are comprised, one or more link performance measures the or each link performance measure relating to a link of which at least one of those end-to-end paths is comprised; then: determining a measure indicative of expected quality of experience in respect of data traversing said particular end-to-end path in dependence on the respective link performance measures determined in respect of the plurality of monitored end-to-end paths. determining a measure indicative of expected quality of experience in respect of data traversing said particular end-to-end path in dependence on the respective link performance measures determined in respect of the plurality of monitored end-to-end paths.