different transmitting powers, different sensitivities and possible bugs many exist. All this definitely influences the subscriber's perceived quality. To perform well in the optimisation phase, network optimisers should work in very close cooperation with other services. The operation and maintenance centre (OMC) team is the most concerned in the sense that all tunings proposed by optimisation personnel should be implemented via the OMC. The roll-out team is the one responsible for all site activity (site re-engineering civil works, site re-hosting from BSC to BSC, TRX redeployment, etc.). The care team ensures alarm monitoring, hardware fault management and maintenance interventions. The operator then comes at another level of interaction to take care of multivendor management, validation and work facilities (access, authorisations, etc.). The success key in such an environment is the application of process where inputs, outputs, procedures and owners are identified. Optimisation engineers have to master each process well and always be ready to adapt the solution because they are often asked for advice on parallel operations taking place at the same time. With the introduction of automation, optimisers are freed to address advanced issues. In more and more complex networks, the optimisation task takes different orientations from before. Effectively, previously the focus was on optimisers' knowledge of one specific technology and of manual optimisation practices. Today, competition in the market of telecommunications reaches a very fierce level where concentration on own technology is not sufficient any more. Today, what is asked from engineers is a multivendor competence. Multivendor competence means being able to address any optimisation or planning issue regardless of the equipment manufacturer. Statistics and Key Performance Indicators Quality of service in telecommunications means the level of usability and reliability of a network and its services. As mentioned before, statistics are the most efficient way