Likely detected fraction of stars in clusters of age t < 10 Myr (blue area and blue lines), 10 Myr < t < 100 Myr (yellow area and yellow lines), and 100 Myr < t < 300 Myr (red area and red lines) as a function of Σ_SFR. Apart from indicating age by colour, age is also shown by a hatch style. The coloured area is bordered by two extreme models: the lower one has β = 2.2 and r_search = 2 pc, the upper one has β = 1.8 and r_search = 5 pc. The models for β = 2.0 and r_search = 2 pc and 5 pc are shown by the dashed lines. The plot shows how increases with the Σ_SFR, and how it decreases with the cluster age. The observational data from Chandar et al. (2017), which are aimed consistently at galaxies both with low SFR and high SFR, are shown by empty squares with their errorbars. Our results are obtained at almost the same time intervals as the observational data (indicated by the colours). The semi-analytic estimate provided by the Γ − Σ_SFR model is shown by the black solid line. We note its disagreement with the observational data for the clusters of any age interval.