However, using clusters at a range of distances suffers not only from the Malmquist bias, but from the Scott effect - the fact that to be recognized as such at great distances, clusters become less and less typical in population. These selection effects are especially damaging for distance-scale problems. If we regard Hubble type as mapping a continuous structural variable, the number of galaxies tells us about the bin widths of the Hubble classes in this variable. Many useful quantities correlate with T, as shown in his Fig. 2. Note that corrections for internal extinction needed to be made.
Later types have bluer colors, indicating a larger relative rate of recent star formation.
I content along the Hubble sequence. All the selection effects fall into determining V m for a given threshold condition, which may be nontrivial. Actually, it always seems to be nontrivial. A value smaller than this implies that there are more objects close to us, which for galaxies normally means that the sample is more incomplete at large distances than we initially assumed. This apparent distribution declines fainter than absolute blue magnitude -21.5, while the space density continues to increase greatly to fainter absolute magnitudes.