The application of quasi-median networks provides an effective tool to check the quality of mtDNA data. Filtering of highly recurrent mutations prior to network analysis is required to simplify the data set and reduce the complexity of the network. The phylogenetic background determines those mutations that need to be filtered. While the traditional EMPOPspeedy filter was based on the worldwide mtDNA phylogeny, haplogroup-specific filters can more effectively highlight potential errors in data of the respective (sub)-continental region. In this study the authors demonstrate the performance of a new, west Eurasian filter EMPOPspeedyWE for the fine-tuned examination of datasets belonging to macrohaplogroup N that constitutes the main portion of mtDNA lineages in Europe. The effects on the resulting network of different database sizes, high-quality and flawed data, as well as the examination of a phylogenetically distant dataset, are presented by examples. The analyses are based on a west Eurasian etalon dataset that was carefully compiled from more than 3,500 control region sequences for network purposes. Both, etalon data and the new filter file, are provided through the EMPOP database (www.empop.org). (Published Abstract)