(a)
(b)
Figure 4: (a) The relation with the best-fit line displayed. The errors are given by the confidence interval. (Figure after Schaefer [157]; see Figure therein. @ AAS. Reproduced with permission.) (b) The correlation with the best fit line. (Figure after Xiao and Schaefer [159]; see Figure therein. @ AAS. Reproduced with permission.)