Deviation of the 21 μm-to-CO flux ratio (blue) and Hα-to-CO flux ratio (black) with respect to the galactic average as a function of aperture sizes for each galaxy. The arrows indicate the typical separation length, λ, at which the two tracers decorrelate. The amplitude of the decorrelation and the separation length are systematically smaller for CO versus 21 μm runs than for CO versus Hα runs, highlighting the greater overlap between CO and 21 μm. The legends indicate the criteria used to select our final sample: (1) a resolution R ≤ 180 pc (2) a clear decorrelation between CO and 21 μm with Δ_min > 0.04 dex.