A method of determining quality of service (QoS) in an IT infrastructure on the basis of QoS-related data received from data sources for these QoS-related data, wherein the data from the data sources regularly arrive in real-time, but data from one or more of the data sources may occasionally be delayed. On the basis of the regular data, a real-time indication of the quality of service is calculated in a first process. This indication is possibly inaccurate since delayed data is not included. After the receipt of delayed data, a delayed, but more accurate indication of the quality of service is calculated in a second concurrent process on the basis of regular information, which comprises at least one of regular data and information derived from it, and the delayed data.