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44 Chapter 1 Sex distribution Sex distribution of FC was reported in 19 studies. Eight of these 19 studies (42%) reported a higher prevalence of FC in girls, 5 studies (26%) reported a higher prevalence in boys and 5 studies (26%) described an equal sex distribution of FC in both sexes. One study reported different sex distributions at 3, 6 and 12 months respectively. 45 In only two studies the reported difference in sex ratio was statistically significant; Wu et al . described a significant higher prevalence in females compared to males (36.1% vs. 29.2%, P = .001), whereas Lewis et al . found a significantly higher prevalence in males (15.5% vs. 10.5%, P = .022). 29,46 After pooling the data based on sex, the prevalence of FC was 8.6% in boys compared to 8.9% in girls (OR 0.99, 95% CI 0.9-1.4). Age distribution Data were pooled for children <4 years ( n =2 studies) and ≥4 years ( n =26 studies) to determine differences between infants/toddlers and children/adolescents as per the Rome criteria (Table 3). Five studies could not be assigned to either of these categories because the age range of the sample crossed the limit of 4 years. Based on this comparison, age was not found to be associated with the prevalence of FC ( P = .22). Prevalence of FNRFI The prevalence of FRNFI ranged from 0.0%-1.8% (Table 2). Pooled prevalence according to the random-effect model was 0.4% (95% CI 0.2-0.6, Supplemental file 4b). The relationship between study characteristics and the prevalence of FRNFI is summarized in Table 3. Geographical distribution Most studies on the prevalence of FNRFI were conducted in Asia (6/15, 40%). As shown in Supplemental file 5, the highest prevalence of FNRFI was found in North-America (1.8%), the lowest pooled prevalence was reported in Central-America (0.1%). Data on the prevalence of FNRFI in Africa and Oceania are currently lacking. Geographical distribution was not associated with the prevalence of FNRFI, although results were borderline significant ( P = .056). Sex distribution Two studies investigated sex-related differences in FNRFI prevalence. Sagawa et al . reported a higher prevalence of FNRFI in girls (F:M ratio 2.8:1, ns) 26 , while Lewis et al . reported a male predominance of FNRFI (F:M ratio 0.6:1, ns). 46 Due to the scarcity of data, no pooled prevalence according to the sex distribution could be calculated.