Venn diagram of the relationship of all samples in CSTAR (green frame). Spectra used are from CSTAR_XPM (purple frame, left in the figure). CSTAR_XPM_G is a cross between CSTAR_XPM and golden sample of carbon stars (purplish-red frame, right in the figure), and CSTAR_XPM_nonG is the complement of CSTAR_XPM and CSTAR_XPM_G. Negative sample (yellow) is selected from CSTAR_XPM_nonG, and positive sample (blue) is selected from CSTAR_XPM_G. Gray ellipse frame indicates range of possible carbon stars.