  While radio emission in quasars can be contributed to by a variety of
processes (involving star forming regions, accretion disk coronas and winds,
and jets), the powering of the radio loudest quasars must involve very strong
jets, presumably launched by the Blandford-Znajek mechanism incorporating the
magnetically arrested disk (MAD) scenario. We focus on the latter and
investigate the dependence of their fraction on redshift. We also examine the
dependence of the radio-loud fraction (RLF) on BH mass ($M_{\rm BH}$) and
Eddington ratio ($\lambda_{\rm Edd}$) while excluding the redshift bias by
narrowing its range. In both these investigations we remove the bias associated
with: (1) the diversity of source selection by constructing two well-defined,
homogeneous samples of quasars (first within $0.7 \leq z < 1.9$, second within
$0.5 \leq z < 0.7$); (2) a strong drop in the RLF of quasars at smaller BH
masses by choosing those with BH masses larger than $10^{8.5} M_{\odot}$. We
confirm some previous results showing the increase in the fraction of
radio-loud quasars with cosmic time and that this trend can be even steeper if
we account for the bias introduced by the dependence of the RLF on BH mass
whereas the bias introduced by the dependence of the RLF on Eddington ratio is
shown to be negligible. Assuming that quasar activities are triggered by galaxy
mergers we argue that such an increase can result from the slower drop with
cosmic time of mixed mergers than of wet mergers.
