Graph 3: A compendium of hard X-ray surveys. XMM surveys are denoted in black, others in grey. Surveys which are (or are planned to be) completely identified are denoted by circles or ellipses, surveys with only partial IDs are denoted by squares and rectangles. Symbol size reflects the typical X-ray source positional accuracy (except for the XMM Serendipitous Survey which is expected to have typical positional accuracy of 5 arcsec as for other XMM surveys). The diagonal solid and dashed lines represent 10,000, 1000 and 100 observations per sample assuming Hasinger et al. (1993) log N-log S with a factor 2 higher normalisation