**Table 2:** Parameters and their references used in the analyses.
Parameter	Label on the SOM	References
Optical identification	ID	Tornikoski et al. (2001); O'Dea et al. (2005); Stanghellini et al. (1993); Snellen et al. (2002a); Orienti et al. (2006a); Hewitt & Burbidge (1993); Jeyakumar et al. (2000); Stanghellini et al. (1998); Impey et al. (1991); de Vries et al. (1997); NED; Xiang et al. (2005); Augusto et al. (2006); Xiang et al. (2006); Snellen et al. (1995); Tornikoski et al. (2000); Wills et al. (1992); Dallacasa et al. (2000)
Redshift	z	O'Dea et al. (1996); Hewitt & Burbidge (1993); Jeyakumar et al. (2000); Véron-Cetty & Véron (2003); de Vries et al. (1997); NED; de Vries et al. (2007); Xiang et al. (2006); O'Dea et al. (2005); Orienti et al. (2006b); Stanghellini et al. (1993); Xiang et al. (2005); Labiano et al. (2007b); Stanghellini et al. (1998); Impey & Tapia (1990); Impey et al. (1991); Snellen et al. (1995); Tinti & de Zotti (2006); Dallacasa et al. (2000)
Size	Size_kpc	O'Dea & Baum (1997); Xiang et al. (2006); Dallacasa et al. (1998); Xiang et al. (2005); Orienti et al. (2006a); Jeyakumar et al. (2000); Augusto et al. (2006); Gupta et al. (2006); Labiano et al. (2007b); Stanghellini et al. (2001); Best et al. (1999); Gurvits et al. (1999)
Power at 5 GHz	P_5 GHz	Calculated as described in the text
Optical polarization	p_opt	Fugmann & Meisenheimer (1988); Impey & Tapia (1990); Visvanathan & Wills (1998); Impey et al. (1991); Marcha et al. (1996); Wills et al. (1992); O'Dea (1998)
Radio polarization	p_radio	Ricci et al. (2004); Zukowski et al. (1999); Steppe et al. (1995); Aller et al. (2003); Homan & Lister (2006)
B magnitude		Siebert et al. (1998); Véron-Cetty & Véron (2003); Labiano et al. (2007b); Dallacasa et al. (2002)
V magnitude		Véron-Cetty & Véron (2003); Dallacasa et al. (2002); O'Dea et al. (1991); Wills et al. (1992); Hewitt & Burbidge (1993); Barvainis et al. (2005); Impey & Tapia (1990); Labiano et al. (2007b); Impey et al. (1991)
R magnitude		O'Dea et al. (1996); Labiano et al. (2007b); Véron-Cetty & Véron (2003); Dallacasa et al. (2002); Stanghellini et al. (1993); O'Dea et al. (1991); Tinti & de Zotti (2006)
I magnitude		de Vries et al. (1995); Xiang et al. (2005); Guainazzi et al. (2006); Stanghellini et al. (1993); Dallacasa et al. (2002); de Vries et al. (2000)
V-R colour	V-R	Dallacasa et al. (2002), and calculated values
B-V colour	B-V	Véron-Cetty & Véron (2003), and calculated values
U-B colour	U-B	Véron-Cetty & Véron (2003), and calculated values
VLBI morphology	-	Xiang et al. (2006); O'Dea et al. (1991); Dallacasa et al. (1998); Lister et al. (2002); Xiang et al. (2005); Orienti et al. (2006a); Augusto et al. (2006); Jeyakumar et al. (2000); Gugliucci et al. (2005); Fey & Charlot (1997)
R-I colour	R-I	O'Dea et al. (1996,1991)
Hydrogen column density in X-rays	N_H_X	Siemiginowska et al. (2003); Siebert et al. (1998); Guainazzi et al. (2006); Bloom et al. (1999); Vink et al. (2006); Elvis et al. (1994)
Power law slope in X-rays	Gamma	Siemiginowska et al. (2003); Siebert et al. (1998); Guainazzi et al. (2006); Vink et al. (2006)
Hydrogen column density 21 cm	N_H_21	Gupta et al. (2006); Orienti et al. (2006b); Pihlström et al. (2003)
O-E colour	O-E	Snellen et al. (2002b)
Variability index	vi	Calculated as described in the text
Number of observations	N_vi	Calculated as described in the text
Rest frame turnover frequency	nu_peak,rest	Calculated as described in the text
Optically thick spectral index	alpha_b	Calculated as described in the text
Optically thin spectral index	alpha_a	Calculated as described in the text
Curvature	Curvature	Calculated as described in the text
Symmetry	Symmetry	Calculated as described in the text
Width of the spectrum	FWHM	Calculated as described in the text

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