Fraction of VLBA-detected sources. Left panel: the fraction of VLBA-detected sources as a function of flux density. Detectable sources have been grouped into bins with limits 100 μJy, N = 0,1,2,... Error bars denote the 68% confidence interval for a small-sample binomial distribution, according to Clopper & Pearson (1934). Whilst at the faintest flux density levels only around 15% have been detected, this fraction rises sharply at a level of around 0.5 mJy, and increases to more than 80% at the 2 mJy level. Right panel: using the same bins as for the left panel, the fraction of detected sources brighter than a bin limit has been calculated. Whilst of all sources brighter than 100 μJy 30% have been detected, the fraction reaches 60% when all sources brighter than 1 mJy are considered. The green line indicates the data of the CDFS from Middelberg et al. (2011a), which had a maximum sensitivity of 55   μJy/beam, compared to just under 20   μJy/beam in our data. All curves are subject to small number statistics, which causes the Lockman Hole data to drop at the bright end and the discrepancy between the CDFS and Lockman Hole data.