log EW(C IV) vs. L_UV (upper panel), Δ(log EW(C IV)) vs. L_UV (lower panel) for Type I quasars. The blue line in the upper panel is the theoretical values of log EW(C IV) from Eq. (1) with the best fitting values of β and γ. Δ(log EW(C IV)) represents the residuals from the subtraction of the observational data and theoretical values. In the lower panel the relationship between Δ(log EW(C IV)) and L_UV indicates the luminosity dependence of the BEff slope.