  Unified schemes of radio sources, which account for different types of radio
AGN in terms of anisotropic radio and optical emission, together with different
orientations of the ejection axis to the line of sight, have been invoked for
many years. Recently, large samples of optical quasars, mainly from the Sloan
Digital Sky Survey, together with large radio samples, such as FIRST, have
become available. These hold the promise of providing more stringent tests of
unified schemes but, compared to previous samples, lack high resolution radio
maps. Nevertheless they have been used to investigate unified schemes, in some
cases yielding results which appear inconsistent with such theories. Here we
investigate using simulations how the selection effects to which such
investigations are subject can influence the conclusions drawn. In particular,
we find that the effects of limited resolution do not allow core-dominated
radio sources to be fully represented in the samples, that the effects of
limited sensitivity systematically exclude some classes of sources and the lack
of deep radio data make it difficult to decide to what extent closely separated
radio sources are associated. Nevertheless, we conclude that relativistic
unified schemes are entirely compatible with the current observational data.
For a sample selected from SDSS and FIRST which includes weak-cored triples we
find that the equivalent width of the [OIII] emission line decreases as
core-dominance increases, as expected, and also that core-dominated quasars are
optically brighter than weak-cored quasars.
