The choice of the appropriate time length of the database of accidents considered is therefore subjective and technology-specific: it should be sufficiently long to provide a number of entries statistically significant but also contain data representative of the existing energy infrastructure in terms of technology development and safety and regulatory standards. An appropriate aggregation level of the results is therefore needed to reflect these differences. For example, researchers from the Paul Scherrer Institute (PSI, Switzerland) aggregate the results in three major groups - European Union member states (27), OECD and non-OECD countries', but the latter group is further disaggregated to reflect specific situations for certain energy technologies (for instance the data for coal in China have been reported separately, since they are significantly higher than those of other countries). In the literature, different approaches have been used with respect to the time frame considered and to the geographical segmentation used: a comparative study (Felder, 2009) pointed out that two major accident-related studies from PSI and Sovacool (Sovacool, 2008 and Hirschberg et al.,