Table 1.

Classification of quality indicators of studies included in the review

	Sampling	Participation rate	Population	Sample size
	1 = Non-random	1<60%	1 = Selected	1<100
	2 = Random	2 ≥ 60%	2 = General	2 ≥ 100
Bagley 1997 [9]	2	2	2	1
Bontempo & D'Augelli 2002 [10]	Not known	2	1	2
Case et al. 2004 [11]*	1	2	1	2
Cochran & Mays (2000a) & Mays, Ross (2004) [12,13]	1	2	2	1
Cochran & Mays (2000b) [14]	1	2	2	1
Cochran et al. 2003 & Mays & Cochran (2001) [15,16]	2	2	2	1
Faulkner et al. (1998) [17]	Not known	1	1	2
Fergusson et al 1999 [18]*	1	2	1	1
Gilman et al. 2001 [19]	2	2	2	2
Gruskin et al. 2001 [20]	2	1	2	2
Herrell et al. 1999 [21]	Not known	2	1	2
Jorm et al. 2002 [22]	2	1	2	2
King et al. 2003 [1]	1	NA	1	2
Mathy 2002 [23]	1	1	1	2
Mathews et al. 2002 [24]	1	1	1	2
McCabe et al. 2003 [25]	2	1	1	2
Mc Cabe et al. 2004 [26]	2	1	1	1
Nawyn et al. 2000 [27]	Not known	1	1	1
Remafedi et al. 1998 [28]	Not known	2	1	2
Robin et al. 2002 [29]	Not known	2	1	2
Russell & Joyner 2001 [30]	Not known	Not known	1	2
Sandfort et al. 2001 & Sandfort, de Graf, Bijl (2003) [31,32]	2	2	2	1
Skegg et al. 2003 [33]*	1	2	1	1
Wichstrom & Hegna 2003 [34]*	1	2	1	2
Drabble et al. 2005 [35]	2	Not known	2	2

Key: * Longitudinal studies; 2 suggests higher quality and than 1