Table 2.

Quality assessment: included studies

Study	Screening Q.1: Address clearly focused issue	Screening Q.2: Cohort acceptably recruited	Exposure accurately measured (min. bias)	Outcome accurately measured (min. bias)	Identified important confounding factors	Account for confounding factors in design/ analysis	Follow-up complete enough	Follow-up long enough	Believable results	Applicable to local population	Fit with available evidence	Total Yes	Total No	Total Can’t tell
Beecham17 (1997)	Yes	Yes	Can’t tell	Yes	No	No	Yes	Yes	Yes	Can’t tell	Can’t tell	6	2	3
Hallam18 (2006)	Yes	Yes	Yes	Yes	No	No	Yes	Yes	Yes	Can’t tell	Can’t tell	7	2	2