Expected fraction of clusters with RHs with steepening frequency (Cassano et al. 2010) ν_s> 600 MHz in the redshift range 0.08 <z< 0.33 (red line and shadowed region). Calculations have been performed for the following choice of model parameters: b = 1.5, ⟨ B ⟩ = 1.9 μG (where B = ⟨ B ⟩ × (M/ ⟨ M ⟩ )^b), and η_t = 0.2 (see Cassano et al. 2012, and referencees therein). The observed fraction of clusters with RHs in the two mass bins is also shown (black points with horizontal error bars). The black points with dashed error bars show f_RH when we exclude USSRH (and candidate USSRH) from the observed sample.