(which is also the colour range of the Bluedisk sample). The estimator is a combination of the NUV$-r$ colour and the mass surface density, reflecting the connection between star formation rate and cold gas content. The difference between the observed and the estimated $\fHI$, which we denote as $\Delta_{C13}\fHI$, measures the excess gas, and can be used to indicate [[H]{}[<span style="font-variant:small-caps;">i</span> ]{}]{}richness. Xiao et al. (in prep) further add the optical concentration parameter and stellar mass to the estimator, giving more control on the internal structure of galaxies. We also use the difference from this 4-parameter (4p) estimator ( $\Delta_{4p}\fHI$) as a measure of [[H]{}[<span style="font-variant:small-caps;">i</span> ]{}]{}richness.

![ [[H]{}[<span style="font-variant:small-caps;">i</span> ]{}]{}mass fraction of the Bluedisk sample (dark grey dots), the HIghMass sample (triangles), and the sample from Lemonias et al. (2014) (L14, diamonds). The dashed line shows the median relation between $\fHI$ and stellar mass for general galaxies, taken from Catinella et al. (2010). The dotted line is the dashed line 0.6 dex upward, showing the original division between the Bluedisk high $\fHI$ and control galaxies.[]{data-label="fig:sample"}](plot/sample.ps){width="8cm"}

Method
======

As discussed in the introduction section, the conformity behaviour of central and bright satellite galaxies can be interpreted in terms of an 