X-ray luminosity function of ECDF-S groups. Black dots show the measurement using the full field, while black crosses show the measurements excluding the central region where there is a low spatial density of groups. Gray crosses show the results from the COSMOS field. Dashed and dotted curves show the local XLF in the Northern and Southern Hemisphere, revealing an effect of sample variance, caused by small volumes probed by RASS at low group luminosities.