Noise standard deviation σ_b,ρ in mK whichensures that relative precisions are better than ρ% (for details see Eq. (25)). Top: analysis of a single line. Bottom: analysis of two lines ¹³CO(1− 0) and ¹³CO(2− 1). The contours for 10 and 100 mK are highlighted because these σ_b values bracket the values reached during typical observations at the IRAM-30 m. For this analysis, Δ_V is fixed at 1.1km s⁻¹, but the computations are done for four different values of σ_V (0.3, 0.6, 1.3,and 2.0 km s⁻¹) and then projected on the (N, T_ex) plane (see text for details).