Table 2

Quality assessment

Cohort studies*	Selection				Comparability		Outcome/exposure			Score
	1	2	3	4	5	6	7	8	9
	Representativeness	Selection	Exposure	Outcome	Most important	Second important	Assessment	Duration of follow-up	Adequacy follow-up
Canova et al22	*	*	*	*		*	*	*	*	8/9
Hestbaek et al23	*	*	*	*				*	*	6/9
Hviid et al31	*	*	*	*		*	*	*	*	8/9
Kemppainen et al24		*		*	*	*	*	*		6/9
Kronman et al25		*	*	*	*	*	*	*		7/9
Oosterloo et al26	*	*	*	*	*	*	*	*		8/9
Örtqvist et al27	*	*	*	*	*	*	*		*	8/9
Salvatore et al32	*	*	*	*		*	*	*		7/9
Dydensborg Sander et al28	*	*	*	*	*	*	*	*	*	9/9
Turco et al29	*	*	*	*	*	*		*	*	8/9
Uusijärvi et al30	*	*		*				*	*	5/9

Case–control studies†	Case definition	Cases	Controls	Definition controls	Most important	Second important	Exposure	Ascertainment	Non-response rate	Score
Bittker and Bell42			*	*	*			*	*	5/9
Canova et al43	*	*	*	*		*	*	*	*	8/9
Jensen et al33	*	*		*				*		4/9
Jensen et al34	*	*	*	*				*	*	6/9
Mårild et al35	*	*	*	*		*	*	*	*	8/9
Myléus et al36	*	*	*	*		*		*	*	7/9
Radano et al37	*	*		*	*	*		*	*	7/9
Shaw et al38	*	*	*	*		*	*	*	*	8/9
Slae et al39	*			*				*		3/9
Virta et al40	*	*	*	*		*	*	*	*	8/9
Witmer et al41		*	*	*	*	*		*	*	7/9

Comparability: most important confounder: IBD and CeD: presence of IBD/ CeD in 1st degree family member, EoE: sex, colics: atopy child and/or family, functional constipation: maternal education/social economic status, abdominal pain: lactose intolerance/cow’s milk allergy. Comparability: second important confounder: IBD: ethnicity and/or age, EoE: presence of other atopic diseases and/or ethnicity, CeD: sex and/or season of birth and/or the presence of other autoimmune diseases, colics: presence of GERD and/or type of feeding and/or being a first child, functional constipation: sex and/or age, abdominal pain: anxiety/depression/stress in the child and/or the parents.
*Cohort studies: 1. representativeness of the exposed cohort, 2. selection of the non-exposed cohort, 3. ascertainment of exposure, 4. demonstration that the outcome of interest was not present at start of the study, 5. comparability of cohorts on the basis of the design or analysis most important factor, 6. comparability of cohorts on the basis of the design or analysis second important factor, 7. assessment of outcome 8. was follow-up long enough for outcomes to occur and 9. adequacy of follow-up of cohort.
†Case–control studies: 1. is the case definition adequate? 2. representativeness of the cases, 3. selection of controls, 4. definition of controls, 5. comparability of cases and controls on the basis of the design or analysis most important factor, 6. comparability of cases and controls on the basis of the design or analysis second important factor, 7. ascertainment of exposure, 8. same method of ascertainment for cases and controls and 9. non-response rate.
CeD, coeliac disease; EoE, eosinophilic oesophagitis; GERD, gastro-oesophageal reflux ; IBD, inflammatory bowel disease.