Table 3.

Methodological characteristics of the studies in the present systematic review.

Authors & Year/Country	Study Design	Sample	Measurement of the Main Variables (SBV/SI-NSSI)	Confounding Factors Assessed	Limitations	NOKC /CASP Quality Assessment (low, moderate, high)
Brunner et al. 2014/ 11 European countries	Cross-sectional, comparative, correlational study	Random sample; mean age: 14.9 years; n= 12,068	Self-reported questionnaires: open-ended questions & psychometric scales	Demographic data; income; family type; immigrant status; religiosity; psychopathology; suicidality; anxiety & depressive symptoms; substance abuse; parenting; social relationship problems & loneliness; quality of parenting & communication with parents; impulsivity	Self-reported data. No longitudinal data, thus the study cannot provide information about causality; no triangulation of data with teachers/ parents/peer nomination reports	Moderate quality
Brunstein Klomek et al. 2016/ 10 European countries	Cross-sectional, correlational study	Random sample; mean age: 14.9 years; n= 11,110	Self-reported questionnaires: open-ended questions & psychometric scales	Demographic data; income; family type; immigrant status; religiosity; psychopathology; suicidality; anxiety & depressive symptoms; substance abuse; parenting & support; social relationship problems, peer support & loneliness; quality of communication with parents; impulsivity; prosocial behaviour	Self-reported data; the cross-sectional nature of the study does not allow assumptions on causality; no triangulation of data with teachers/ parents/ peer nomination reports	Moderate quality
Claes et al. 2015/ Belgium and the Netherlands	Cross-sectional & correlational study	Convenience sample; mean age: 15.56 years; n= 785	Self- reported questionnaires, psychometric scales	Depressive symptoms; perceived parental support; age; gender; victimization.	Self-reported data; no triangulation of data with teacher/ parent/ peer nomination reports; only the presence/absence of NSSI was assessed; data were gathered at one point in time; no conclusions on causality; important confounders were not assessed, i.e., impulsivity, drug abuse, self-esteem, mental health problems	Moderate quality
Elgar et al. 2014/USA	Cross-sectional, observational & correlational study	Random sample; mean age 15.0; n=18,834	Anonymous, self-reported, electronically distributed questionnaires: psychometric scales & open-ended questions	Cyber bullying; victimization; anxiety & depressive symptoms; self-harm & suicidal behaviour; physical fighting & vandalizing; substance misuse (alcohol & legal & illegal drugs); family communication/ support; household income; age; gender	Self-reported data; no triangulation of data with teacher/ parent/ peer nomination reports; cross-sectional design, thus no conclusions on causality	Moderate quality
Espelage & Holt 2013/ USA	Cross-sectional study	Random sample; median age 12.3 (range: 10-13); n=661	Anonymous, self-reported questionnaires, Psychometric scales	Anxiety & depressive symptoms; delinquency; suicidal ideation; gender; grade; race	Self-reported data; no triangulation of data with teachers/ parents/ peer nomination reports; cross-sectional design, thus no conclusions on causality; important confounders were not included, e.g. substance use.	Moderate quality
Ford et al. 2017/Australia	Cross-sectional study	Random sample; median age NR (range: 14-15); n=2304	Anonymous, self-reported questionnaires, psychometric scales, face-to-face interviews & computer-assisted interviews.	Gender, household type & income; language spoken in home; parents' education; Aborigin/ Torres Strait	Cross-sectional design, thus no conclusions on causality; important confounders were not assessed, i.e. substance misuse, self-esteem, impulsivity	High quality
Garish & Wlilson 2010/ New Zealand	Cross-sectional & correlational, exploratory study.	Convenience sample; mean age: 16.67 years; n=325	Anonymous, self-reported questionnaires: psychometric scales & open- ended questions	Depressive symptoms; alexithymia; gender	Self-reported data & recall bias; no triangulation of data with teacher/ parent/ peer nomination reports; cross-sectional design, thus no conclusions on causality; important confounders were not assessed, i.e. substance misuse, self-esteem, impulsivity; generalizability limited to adolescents of European origin with a high socioeconomic status.	Moderate quality
Giletta et al. 2012/ USA, Italy, The Netherlands	Cross-sectional & correlational study	Convenience sample; mean age=15.7 years; n=1,862	Anonymous, self-reported questionnaires: psychometric scales & open- ended questions	Age, gender, ethnicity and parents' educational level; depressive symptoms; family & peer related loneliness; peer preference (interpersonal stressors); substance use	Self-reported data; no triangulation of data with teachers/ parents/ peer nomination reports; cross-sectional design, thus no conclusions on causality; important confounders were not assessed, i.e. self-esteem, impulsivity; low response rate in the subgroups; convenience sample	Moderate quality
Gower & Borowsky 2013/ USA	Cross-sectional study	Convenience sample; mean age: NR; n=128,681	Self-reported questionnaires, psychometric scales	Age; gender; ethnicity; impulsivity; suicidality; personal & parental mental health problems & substance use; emotional distress; family conflict & running away; skipped school; negative self-concept; religious activities; supportive social network; conduct problems; depressive and anxiety symptoms; emotional & physical domestic violence; physical & sexual abuse; witness to domestic violence; parent connectedness; academic performance; family structure & income; residency	Self-reported data; no triangulation of data with teacher/ parent/ peer nomination reports; no causality	Moderate quality
Hay & Meldrum, 2010/ USA	Cross-sectional, noncomparative, correlational study	Convenience sample; mean age: 15 years; n=424	Self-reported questionnaires: open-ended questions & psychometric scales	Age; gender; ethnicity/origin; family type; school performance; impulsivity; authoritative parenting	No triangulation of data with teacher/ parent/ peer nomination reports; no causality; convenience sample; no assessment of mental health variables as confounders	Moderate quality
Jantzer et al. 2015/ Germany	Cross-sectional, noncomparative, correlational study	Entire target population; mean age: 12.8 years; n= 647	Self-reported questionnaires: open-ended questions & psychometric scales	Age; gender; suicidal behaviour; grade; parental monitoring	No triangulation of data with teacher/ parent/ peer nomination reports; no causality; convenience sample; no assessment of important confounders, e.g. impulsivity; self-esteem, etc.	Low quality
McMahon et al. 2010/Ireland	Cross-sectional, noncomparative, correlational study	Random sample; mean age: 16 years; n= 3,881	Self-reported questionnaires: open-ended questions & psychometric scales	Anxiety; impulsivity; self-esteem; DSH of a friend/ family member; drug use; sexual abuse; friendship difficulties; fights with parents; dysfunctional school performance	No triangulation of data with teacher/ parent/ peer nomination reports; no causality assessment; no assessment of important confounders, e.g. substance use; no assessment of social support variables; exclusion of those who did not describe DSH behaviour (risk of underestimation of the prevalence)	Moderate quality
Noble et al. 2011/ USA	Cross-sectional, comparative study	Purposeful (matched groups) sample; mean age: 14.9 years; n= 1,276	Self-reported questionnaires: open-ended questions & psychometric scales	Perceived trust in school context (trust in students/teachers/administration/school counsellor) & safety (missed days due to feeling unsafe; carrying a weapon/ threatened/being bullied at school)	No triangulation of data with teacher/ parent/ peer nomination reports; no causality assessment; convenience sample; no assessment of mental health variables as confounders; social support variables were not included	Moderate quality
O'Connor et al. 2009/Scotland, UK	Cross-sectional, noncomparative, correlational study	Random sample; mean age: 15 years; n= 2,008	Self-reported questionnaires: open-ended questions & psychometric scales	Anxiety & depression symptoms; impulsivity; self-esteem; DSH of a friend/ family member; drug use; sexual abuse; friendship difficulties; fights with parents; dysfunctional school performance	No triangulation of data with teacher/ parent/peer nomination reports; no causality assessment; no assessment of social support variables	Moderate quality
O'Connor et al. 2014/ Northern Ireland	Cross-sectional, noncomparative, correlational study	Random sample; mean age: 15 years; n= 3,596	Self-reported questionnaires: open-ended questions & psychometric scales	Anxiety; depression; impulsivity; self-esteem; DSH of a friend/ family member; drug/alcohol use; sexual/physical abuse; sexual orientation concerns; exercising; living with both parents; exposure to internet/TV DSH images; exposure to difficulties related to “The Troubles”	No triangulation of data with teacher/ parents/ peer nomination reports; no causality assessment; no assessment of social support variables	Moderate quality
Thomas et al. 2017/ Australia	Cross-sectional, correlational study	Random, nationally representative sample; mean age: 14.6 years; n=2967	Self-reported questionnaires: open-ended questions & psychometric scales Face-to-face structured interview for parents/carers	Age, gender	Cross-sectional design, thus no conclusions on causality; important confounders were not assessed, i.e. substance misuse, self-esteem, impulsivity, etc; over-representation of socially/income advantaged families	High quality
Fisher et al. 2012/ UK	Longitudinal birth cohort, comparative study	Birth cohort sample; mean age: NR; n= 2,232	Clinical interviews of mothers/children /teachers: open-ended questions & psychometric scales	Exposure to physical/ sexual maltreatment; anxiety symptoms; depressive symptoms; withdrawn, aggressive & delinquent behaviour; IQ;	The small number of children who engaged in self-injurious behaviour led to biased estimations about the size of the association between the main variables; no inclusion of important confounders, i.e. substance use; parenting & social support variables were not included	High quality
Garisch & Wilson, 2015/New Zealand	Prospective study with measurement at two time points	Random sample; mean age: 16.34(T1) -16.45(T2) years; n=830	Self-report questionnaires	Gender; anxiety & depressive symptoms; self-esteem; alexithymia; adaptive emotional response; resilience; impulsivity; physical & sexual abuse history; substance abuse; sexuality concerns; mindfulness	Moderate internal consistency & test-rest reliability of the instruments applied	High quality
Giletta et al. 2015/China	Prospective cohort comparative study	Random sample; 10th grade (mean age: 16 years); n=565	Self-reported questionnaires & peer nominated data	Gender; suicidal ideation; depressive symptoms; stressful peer experiences/ type and quality of friendships; friend support	Peer nominated data regarding overt and relational school bullying victimization, excluding subjective experiences; the low NSSI/ SI trajectory group included those reporting no or very few episodes, thus the degree to which school bullying victimization differentiated those who engaged in SITB from those who did not at all was not clearly reported; no assessment of substance use as a confounder	High quality
Heilbron & Prinstein 2010/ USA	Longitudinal, population-based comparative cohort study	Random sample; mean age: 12.6 years; n= 493	Clinical interviews with students and peers	Peer status/popularity; depressive symptoms; gender; suicidal ideation	Only partially ethnically diverse sample; no inclusion of important confounders in the analysis/study design, i.e., substance abuse, impulsivity, suicidal behaviour (suicide attempts & plans); parenting/ social support was not assessed; small group sizes in the internal comparisons	High quality
Lereya et al. 2015/ USA & UK	Comparative study of longitudinal birth cohort & population-based data.	Cohort sample; mean age: NR; n= 5,446	Self-reported postal questionnaires about self-harm variables from the adolescents at the age of 16-17 years; face-to-face interviews with the children at the age of 8 & 10 years/mothers/ teachers about predictor variables	Gender; ethnicity; parents' educational level & marital status; parental mental health problems; parental stress; family conflict; preschool maladaptive parenting (hitting, shouting, hostility); conduct problems; hyperactivity; depressive and anxiety symptoms; emotional & physical domestic violence; borderline personality disorder symptoms; sexual abuse	Self-reported data & recall bias; face-to-face interviews & embarrassment bias	High quality
Lereya et al. 2013/ UK	Longitudinal birth cohort, comparative study	Birth cohort sample; mean age: NR; n= 4,810	Self-reported postal questionnaires about self-harm variables from the adolescents at the age of 16-17 years; face-to-face interviews with the children at the age of 8 & 10 years/mothers/ teachers about predictor variables.	Gender; preschool maladaptive parenting (hitting, shouting, hostility); conduct problems; hyperactivity; depressive symptoms; emotional & physical domestic violence; borderline personality disorder symptoms	Self-reported self-harm & recall bias; no assessment of social support; face-to-face interviews & embarrassment bias.	High quality

SBV: school bullying victimization; NSSI: non-suicidal self-injury; SI: self-injury.