. 2021 Mar 3;5(1):e001028. doi: 10.1136/bmjpo-2021-001028

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.