Compactness as a function of the arcsec-scale 1.4 GHz flux density. VLBA-detected IFRS are shown by crosses and the 6.75σ upper limits on the compactness of VLBA-undetected IFRS are shown by black arrows. The grey horizontal lines represent – from top to bottom – the mean compactness of all VLBA-detected IFRS, of all VLBA-observed IFRS, and all VLBA-undetected IFRS, respectively. The latter ones represent upper limits as indicated by grey arrows. The black line shows the minimal detectable compactness depending on the arcsec-scale flux density, based on an mean detection sensitivity of 450 μJy in our VLBA observations.