Table 2. Assessment of the methodological quality of the included studies.

External validity		YES	NO	UNCLEAR	NOT APPLICABLE
1.	Are the study participants representative of the population under examination?	(+)	(-)	(0)	(/)
Measurement bias
	Were valid and reliable measurement used to assess:
2.	…the protective factors?	(+)	(-)	(0)	(/)
3.	…parental drug abuse disorder or addiction?	(+)	(-)	(0)	(/)
Attrition bias (cohort studies)
4.	Was the dropout rate in both groups acceptable (≤ 20%)?	(+)	(-)	(0)	(/)
Selection bias (case-control studies)
5.	Were the participants in the groups comparable with respect to possible confounders?	(+)	(-)	(0)	(/)