Three studies used a longitudinal design. The follow-up duration ranged between 9 months and 11 years. Across the 3 samples, bullied children were found to have a significantly higher risk for headache than non-bullied agemates were (OR = 2.10, 95% CI = 1.19-3.71, Z = 2.57, P = .01). Figure 2 shows the forest plot for this meta-analysis. Studies were not completely homogeneous (Q = 4.11, P = .13, I2 = 51.37%). Across the 17 samples that were included in the cross-sectional studies,

bullied children were found to have a significantly higher risk for headache than were non-bullied peers (OR = 2.00, 95% CI = 1.70-2.35, Z = 8.43, P < .001). Figure 3 shows the forest plot for this meta-analysis. Effect sizes within this group of studies were not homogeneous (Q = 65.64, selleck chemicals llc P < .001, I2 = 75.63%). Moderator analyses with gender composition of the sample, number of confounders, and geographical location were performed to explore possible explanations for heterogeneity in the

effect sizes across cross-sectional studies. Peptide 17 The proportion of girls in the sample was available for 15 out of the 17 cross-sectional studies, and it was used as a continuous predictor in a weighted mixed-effects meta-regression. Results indicated that the magnitude of the effect size significantly decreased with the increase of the number of female participants in the study sample (B = −.06, 95% CI: −.07 to −.04, P < .001). Conversely, the number of confounders considered in the study (range: 0-6) did not moderate the magnitude of the effect (B = .005, 95% CI: −.04 to .05, P = .82). Also the study's geographical location (coded as Europe vs other countries) was not a significant moderator (k = 11, OR = 2.03, 95% CI: 1.59-2.60, and k = 5, OR = 2.00, 95% CI: 1.32-3.02, respectively; Q = .48, P = .79). Finally, consistent with the MOOSE guidelines[27] and to the former meta-analyses,[22, 23] a sensitivity analysis others was performed based on 2 aspects of study quality (beyond those required as inclusion criteria): (1)

the use of a randomized sampling design or a whole population of students; and (2) a good response rate (>80%). Thirteen cross-sectional studies satisfied both criteria. We then performed a separate meta-analysis of this subgroup of studies, and the resulting OR and confidence interval was OR = 1.90, 95% CI = 1.61-2.25. Another sensitivity analysis was performed with the 13 studies that used only self-report questionnaires to gather data from participants. Estimated OR was 1.87, with a 95% CI ranging from 1.57 to 2.23. No evidence of publication bias was present. Kendall’s tau was 0.13 with 2-tailed P = .44. An additional 253 studies with null effect sizes would be needed to attenuate the effect size to a negligible value (“5k + 10” benchmark = 110). The results of this meta-analysis confirmed that bullied youths are about twice more likely than non-bullied agemates to suffer from headache. Same results were found both in longitudinal and cross-sectional studies.