(within a given sample) that are either particularly gas-rich or gas-poor (e.g. Catinella et al. 2012). Similarly, Cortese et al. (2011) demonstrate that the distance from the gas fraction plane is a good alternative to the classic ‘HI deficiency’ parameter (Haynes & Giovanelli 1984). However, extreme caution must be applied in using scaling relations to *estimate* , due to the inherently biased nature of the samples from which they are (currently) constructed. For example, the application of scaling relations derived from the blind, but shallow, ALFALFA data tends to over-estimate  in deeper, or targeted surveys (e.g., Huang et al. 2012). Problems may persist even when precautions are taken to identify galaxies with broadly similar properties as the calibration sample. For example, Zhang et al. (2009) identify a sample of star-forming galaxies from which they determine their scaling relation. It might therefore be expected that the distinction between star-forming and quiescent galaxies may help to identify samples of galaxies appropriate for the application of the Zhang et al. (2009) parametrization. However, Huang et al. (2012) show that the Zhang et al. (2009) relation is not a good predictor of  in ALFALFA, despite the domination of star-forming galaxies in the latter. Therefore, 