This document discusses profiling Big Data sources to assess their selectivity. It analyzes a random sample of 1,000 Dutch Twitter users to determine gender selectivity. Several methods are used to infer gender from profile elements: (1) First names are analyzed using a Dutch name database, (2) Bios and tweets are examined for gendered language, (3) Pictures are processed with face recognition software. Overall results show first names provided the highest diagnostic odds ratio for determining gender, while profile pictures provided the lowest. The study aims to develop clever ways to combine these methods for more accurate gender profiling of social media users.