Comparison of the mass-redshift sampling of the ECDF-S (filled black circles) and COSMOS (filled grey circles) X-ray group samples. Definition of mass with with respect to the critical density. The ECDF-S groups extend to much lower masses, while occupying a similar redshift range. An improvement in the mass sensitivity of the survey scales as exposure to the power of 3.3, so 30 times deeper data in ECDF-S results in a 3 times better mass limit.