Comparison of the optical properties of X-ray-detected galaxies and all optically selected (SDSS) galaxies on or near ChaMP fields/chips. Left: histograms of redshift ( z), apparent r-band magnitude ( m _r ), absolute r-band magnitude ( M _r ), and the concentration index ( C) for the full samples of SDSS galaxies on/near ChaMP fields and the ChaMP detections; median values are indicated by the vertical bars. Right: individual measurements of z, m _r , M _r , and C are shown separately per galaxy spectral type; both average (squares) and median (circles) values are indicated here, for all types of objects. The bias caused by the X-ray detection criterion is clearly weak; the few X-ray-detected H ii's dominate the observed difference in r and M _r distributions.