mass range are only $4.1 \pm 2.0$% younger, with $z_{*} =
1.95_{-0.08}^{+0.10}$. (These results assume that the IMF has a “standard” form and that the progenitor bias described by van Dokkum & Franx (2001) does not depend on environment; see vDvdM06 for details).

The success and popularity of FP studies can be attributed at least in part to the relative ease with which such studies can be performed. Only global galaxy properties need to be measured, and modeling of the internal structure of the sample galaxies is not required. However, there are important caveats. In particular, values of $d \log (M/L) / dz$ determined from FP evolution are correct only if many simplifying assumptions are satisfied. Any potential evolution of global galaxy properties other than $M/L$ (i.e., $\sigma_{\rm
eff}$, $r_{\rm eff}$ and $\Sigma_{\rm eff} \equiv (M/L)\times I_{\rm
eff} (r)$, the latter quantity being the average projected surface mass density inside $r_{\rm eff}$) must either be absent or be such so as to not affect the inferred $M/L$ evolution. Also, galaxies must change homologously with redshift, if at all; they must not evolve in quantities such as the shape of their three-dimensional contours, the shape of their density profile with radius, 