caption: Contour plot depicting the parameter space within which it is optimal to perform additional ingroup taxon sampling relative to increased character sampling and vice versa. When rates of character evolution (λ) are fast, and when ingroup taxa may be sampled that branch close to the internode of interest (<< T), then sampling incorporated characters from novel ingroup taxa is predicted to be more cost effective than sampling novel characters from incorporated taxa. The rate c is the optimal rate of evolution for a character to be sampled via additional character sampling of the 4 taxa composing the nominal quartet. As → 0, the rate t is the optimal rate of evolution for a character to be evolving at if it is to be sampled via additional ingroup taxon sampling.