Table 3.

Methodological quality of included trials*

	Descriptive†				Statistical				Internal Validity								Total Validity
Study	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	?	+
Binder, 2002	+	+	+	+	-	+	+	+	+	+	?	?	+	-	+	+	2	6
Boshuizen, 2005	+	+	-	+	-	+	+	+	?	?	?	+	+	-	+	+	3	5
Chandler, 1998	+	+	-	+	-	+	-	+	?	+	?	?	+	+	+	+	3	6
Chin A Paw, 2001	+	+	+	+	-	+	+	+	?	+	?	+	-	-	+	+	2	5
Gill, 2002	+	+	+	+	-	+	+	+	?	+	?	+	+	+	+	+	2	7
King, 2002	+	+	+	+	-	+	+	+	?	+	?	-	+	-	+	+	2	5
Kretser, 2003	+	+	-	+	-	+	+	?	-	?	?	?	?	+	+	+	5	3
Payette, 2002	+	+	-	+	-	+	+	+	?	+	?	-	+	+	+	+	2	6
Timonen, 2004	+	+	-	+	+	+	-	+	?	+	?	+	?	+	+	+	3	6
Worm, 2001	+	-	-	+	-	+	-	+	-	?	?	+	?	+	+	+	3	5

* '+' criterion fulfilled; '-' criterion not fulfilled; and '?' data not provided or unclear.

^† see table 2 for a detailed description of the items.