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Using a sample of optically-selected quasars from the Sloan Digital SkySurvey, we have determined the radio-loud fraction (RLF) of quasars as afunction of redshift and optical luminosity. The sample contains more than30,000 objects and spans a redshift range of 0<z<5 and a luminosity range of-30<M_i<-22. We use both the radio-to-optical flux ratio (R parameter) andradio luminosity to define radio-loud quasars. After breaking the correlationbetween redshift and luminosity due to the flux-limited nature of the sample,we find that the RLF of quasars decreases with increasing redshift anddecreasing luminosity. The relation can be described in the form oflog(RLF/(1-RLF)) = b_0 + b_z log(1+z) + b_M (M_{2500}+26), where M_{2500} isthe absolute magnitude at rest-frame 2500A, and b_z, b_M<0. When using R>10 todefine radio-loud quasars, we find that b_0=-0.132+/-0.116, b_z=-2.052+/-0.261,and b_M=-0.183+/-0.025. The RLF at z=0.5 declines from 24.3% to 5.6% asluminosity decreases from M_{2500}=-26 to M_{2500}=-22, and the RLF atM_{2500}=-26 declines from 24.3% to 4.1% as redshift increases from 0.5 to 3,suggesting that the RLF is a strong function of both redshift and luminosity.We also examine the impact of flux-related selection effects on the RLFdetermination using a series of tests, and find that the dependence of the RLFon redshift and luminosity is highly likely to be physical, and the selectioneffects we considered are not responsible for the dependence.Comment: 23 pages, 8 figures. Accepted for publication in Ap

The Radio‐Loud Fraction of Quasars is a Strong Function of Redshift and Optical Luminosity