Table 1.

Quality appraisal checklist based on Cross and Hare (2013)

Criteria	Quality rating
	0	1	2
1. Sample size?	Fewer than 15 participants	15+ participants	30+ participants
2. Recruitment?	Participants selected by clinicians or researcher	Participants recruited either through community outreach, a charity, school, or medical clinic	Multiple methods, multiple clinics, school, or multiple charities are used for recruitment
3. Diagnosis? a	Diagnosis not confirmed	Diagnosis based on non-expert opinion or physical features	Confirmed clinical diagnosis or appropriate genetic/enzyme testing
4. Comparison group? b	No comparison group	Comparison between non-genetically distinct group	Genetically or intellectually distinct comparison group
5. Methodology?	No validated or standardised measures are used	Use a validated and/or standardised assessment tool	Multiple standardised and/or validated measures are used alongside new measure, observations, or other methodology.
6. Appropriate statistics/ comparisons?	Data not analysed	Descriptive statistics are used	Appropriate comparative/correlative statistics are reported

^{a, b} Quality rating options adapted to be applicable for both non-syndromic and syndromic intellectual disability groups