A method of calculating a performance indicator indicating a performance of a service at one of a plurality of mobile terminals (10; 10a, 10b, 10c) connected to a network, the method comprising the steps of: - receiving in a network entity a set of infrastructure performance indicators, each infrastructure performance indicator being determined for a part of the network conveying data to one or more of said mobile terminals (10; 10a, 10b, 10c, 11; 11a, 11b) connected to the network; - determining one of local performance indicators, each local performance indicator being determined locally at one mobile terminal of a subset of the plurality of mobile terminals (11; 11a, 11b) connected to the network and provided with a capability to determine one or more local performance indicators, said step comprising the steps of: - acquiring screenshot data displayed on one mobile terminal (11; 11a, 11b) of said subset of mobile terminals; - processing said acquired screenshot data for determining the presence of a screen display element (20a, 20b, 20c, 20d, 20e, 20f) indicating the performance of the service at said one mobile terminal; - determining at least one local performance indicator of the set of local performance indicators using said screen display element (20a, 20b, 20c, 20d, 20e, 20f); the method further comprising: - receiving in the network entity a set of the local performance indicators; - determining a correlation between the set of infrastructure performance indicators and the set of local performance indicators; and - calculating said performance indicator indicating the performance of the service at one or more of the plurality of mobile terminals (10; 10a, 10b, 10c) not equipped with any indicator determining functions, using said correlation and one or more infrastructure performance indicators of said set of infrastructure performance indicators. - receiving in a network entity a set of infrastructure performance indicators, each infrastructure performance indicator being determined for a part of the network conveying data to one or more of said mobile terminals (10; 10a, 10b, 10c, 11; 11a, 11b) connected to the network; - determining one of local performance indicators, each local performance indicator being determined locally at one mobile terminal of a subset of the plurality of mobile terminals (11; 11a, 11b) connected to the network and provided with a capability to determine one or more local performance indicators, said step comprising the steps of: - acquiring screenshot data displayed on one mobile terminal (11; 11a, 11b) of said subset of mobile terminals; - processing said acquired screenshot data for determining the presence of a screen display element (20a, 20b, 20c, 20d, 20e, 20f) indicating the performance of the service at said one mobile terminal; - determining at least one local performance indicator of the set of local performance indicators using said screen display element (20a, 20b, 20c, 20d, 20e, 20f); - acquiring screenshot data displayed on one mobile terminal (11; 11a, 11b) of said subset of mobile terminals; - processing said acquired screenshot data for determining the presence of a screen display element (20a, 20b, 20c, 20d, 20e, 20f) indicating the performance of the service at said one mobile terminal; - determining at least one local performance indicator of the set of local performance indicators using said screen display element (20a, 20b, 20c, 20d, 20e, 20f); - receiving in the network entity a set of the local performance indicators; - determining a correlation between the set of infrastructure performance indicators and the set of local performance indicators; and - calculating said performance indicator indicating the performance of the service at one or more of the plurality of mobile terminals (10; 10a, 10b, 10c) not equipped with any indicator determining functions, using said correlation and one or more infrastructure performance indicators of said set of infrastructure performance indicators.