Hα concentration index, C_Hα, plotted against R-band concentration index, C₃₀, for the full field comparison sample and the cluster and supercluster ELG objects. Here the solid line is the best fit to the full field data and has equation C_Hα = 1.085C₃₀ − 0.094. The dotted line in the lower panel shows an alternative best fit line, with equation C_Hα = 1.702c₃₀ − 0.257, calculated from only the cluster ELG data. This shows that the relationship between C₃₀ and C_Hα is noticeably steeper for the cluster sample than it is for the field. Typical error bars are shown in the upper-right corner of each frame.