Number of 21-cm absorbers per unit redshift range, n₂₁(z), for   =  0.3 km s^-1. Open circles are for no correction for partial coverage. Filled circles correspond to n₂₁ after the optical depths have been corrected for partial coverage assuming f_c = c_f. The n₂₁ based on DLAs at z > 2 are plotted as squares and crosses with the latter corresponding to values after correcting for partial coverage. The curve for non-evolving population of 21-cm absorbers normalized at n₂₁ (z = 1.3) is also plotted. Lines and dashed areas show the number of absorbers per unit redshift for DLAs (Rao et al. 2006,dotted line) and strong Mg ii absorbers (Prochter et al. 2006,dashed line). Triangles are the number per unit redshift of DLAs measured from the sample of Noterdaeme et al. (2009b).