Table 4.

A quality scoring system for cohort studies

First author	P (2), R (1)	Was exposure assessment the same for all participants?	Was outcome assessment the same for all participants?	Were outcome assessors blind to exposure status?	Were the number of and reasons for study withdrawals reported?
		Yes (2), Partial (1), No (0)	Yes (2), Partial (1), No (0)	Yes (2), Partial (1), No (0)	Yes (2), Partial (1), No (0)
Lau 2001	1	1	2	1	1
Kessler 1999	2	1	2	1	2
Wilkinson 2004	2	1	2	1	2
Garcia-Aymerich 2003	2	2	2	1	2
Groenwegn 2003	2	1	2	1	2
Soler 2005	2	2	0	1	2
Wang 2005	1	1	1	0	2
Gudmundsson 2005	2	1	1	1	2
Connolly 2006	2	1	1	2	1

R = retrospective, P = prospective