as in Fig. \[Fig2\], but the derived age is 2.10 Gyr), the middle panel shows the SED fitting for one example of HSBGs (same as in Fig. \[Fig2\] but the derived age is 1.96 Gyr), and the right panel shows the histogram distributions of the derived ages of LSBGs (shade region) and HSBGs (dashed-line) with bin of 0.2 Gyr. []{data-label="Fig3"}](ms471fig3c.eps "fig:"){width="4.6cm" height="4.2cm"}

Incompleteness effects
======================

It is important to test the completeness of the sample, and discuss its effect on the derived ages of the galaxies.

The volume-limited sub-sample
-----------------------------

The incompleteness of the sample could affect the derived results of our analysis, thus it is important to test. Zhong et al. (2008) has carefully discussed the incompleteness of the sample of LSBGs. In their Sect. 2, the good completeness of the SDSS MGS has been discussed. The MGS is a spectroscopic sample selected from the SDSS photometric data. The completeness for objects with spectroscopy observations is high, exceeding 99%, and the fraction of galaxies eliminated by surface brightness cut is very small ($\sim0.1$%). Relative to all the SDSS targets, the SDSS spectroscopic survey is 90% complete (Blanton et al. 2003; Hogg et al. 2004; Strauss et al. 2002; McIntosh 