Variation in the R₂₁ ratios with Galactocentric distance, R_g. The orange values refer to 1 kpc bins; the straight orange lines connecting them give the unweighted mean value within a bin, and the dispersion indicates the propagated uncertainty. The green values, weighted by the square of the uncertainties, correspond to 1 kpc bins. Values of R₂₁ > 2.2 are not shown here, and values of R_g < 14 kpc are obtained from previous measurements (Handa et al. 1993, 1999; Sakamoto et al. 1995; Hasegawa 1997; Usuda et al. 1999). The horizontal dotted black line denotes a value of R₂₁ = 0.7.