Is there a deficit of S0 galaxies at intermediate redshift?

Two contradictory results on the evolution of SO galaxies now exist in the recent literature: either SO's are old (z(formation) > 2) and are evolving passively or most of them form at z < 0.5, as implied by the deficit of SO's in intermediate-redshift (z similar to 0.5) clusters. The resolution of this controversy may be that the apparent deficit of SO's has been derived from a quantity-the E-to-SO ratio-that is prone to morphological classification errors. Once all sources of error are taken into account, the E-to-SO ratios of clusters at different redshifts are fully compatible, and no additional creation of SO's at z < 0.5 is required by the data. Furthermore, there is no deficit at all of SO's in the intermediate-redshift cluster for which we have morphological types of very high quality and thus derive an E-to-SO ratio with a small error.