ITU-T Recommendation G.955 [1] contains minimum requirements for the performance between reference points S and R of the optical path, in terms of attenuation and dispersion, for a digital line system on single-mode optical fibre. When combined with ITU-T Recommendation G.652 [2] on the fibre characteristics, these figures can be used in a worst-case design for the maximum repeater section length. This approach employs worst-case values for all transmission parameters resulting in a repeater section design which guarantees attenuation and dispersion penalties between the S and R reference points which are lower the specified limits in 100 % of all cases. Studies carried out in several countries, show that a worst-case design approach can lead to significant cost penalties for long-haul systems. Recently statistical and semi-statistical approaches have been considered in designing the maximum repeater section length, taking account of the expected statistical variations in the transmission parameters of equipment and cable (e.g. cable attenuation, splice attenuation, transmitted power, etc.). These approaches are based on designing the maximum repeater section length for a high probability (e.g. 99 %) of each repeater section having an adequate system margin over the lifetime of the system. Statistical and semi-statistical approaches achieve significant cost-saving through reductions in the number of the repeaters for a long-haul system, but admit a pre determined probability of insufficient optical path allowances between reference points S and R. The cost savings are substantial for remote power feeding systems where local electrical power plant does not exist. This ETR contains definitions of the different design approaches, statistical dati forms and numerical examples for statistical design for fibre optics systems. The use of this ETR is at the discretion of the reader.