The model assumes that the progenitors of early-type galaxies formed stars from time t_start to t_stop, and that some time after the end of star formation they are morphologically classified as early-type:

For an individual galaxy, the time of morphological transformation is t_trans = t_stop + Δt, with Δt the time interval between cessation of star formation and morphological classification as early-type. The fraction of present-day early-type galaxies that have formed by redshift z is therefore a function of τ (which determines the distribution of t_stop) and Δt.

In the field next to this Help link, the calculator provides the fraction of present-day early-type galaxies that have formed by z=1. This fraction is calculated from the values of τ and Δt entered on the previous lines. Note that the output table gives the early-type galaxy fraction as a function of redshift, for the full range z=0-2.