Illustration of the distance threshold. The distance threshold is chosen such that it divides the dataset into two separate groups (red and blue groups). It is clear that there is no unique best threshold. Any threshold between 10 and 14 or between 16 and 21 has only either one false negative or one false positive. However, there is no perfect threshold that separates the datasets into two pure groups.
Ghalwash and Obradovic BMC Bioinformatics 2012 13:195 doi:10.1186/1471-2105-13-195