Top: observed surface density profiles of various SFR tracers (see text); the dotted curve – with no error bars – is the analytical form suggested by Green (2013) and it is here adopted as representative of the MW SFR profile. Middle: theoretical or empirical SF rates compared to the adopted profile of SFR tracers (the dotted curve from the upper panel); all profiles are normalised to the same value in R₀ = 8 kpc. Bottom: ratio of the theoretical or empirical profiles to the adopted observed one.