Table 1. Eligibility criteria.

	Inclusion criteria	Exclusion criteria
Population (P)	Studies involving human participants who are adults aged ≥ 18 years.	Studies in people < 18 years of age, studies that combined three age groups: children, adolescents, and adults, but did not but did not analyse the data explicitly for each group.
Exposure (E)	Studies on DS and Trisomy 21.	Studies of other chromosomal disorders or other IDDs with no known cause and adults with DS with dementia.
Outcome (O)	Studies on QoL and well-being of adults with DS using self-report and/or proxy reports. Studies that combined children, family members/caregivers and adults with DS, but QoL data from adults with DS were collected and analysed separately. Studies that combined IDD and DS but explicitly collected and analysed QoL data for adults with DS separately.	Studies on FQoL, HRQoL and studies that combined IDD and DS but did not separately analyse and report QoL data from adults with DS.
Study design	Quantitative (cross-sectional and cohort or longitudinal studies), qualitative (interviews, focus groups, case studies, image-based methods) and mixed methods studies.	Clinical trials evaluating the potency of a drug or medical intervention in relation to a clinical outcome.
Other	Studies in English language only.	Studies published in languages other than English due to a lack of resources for translating data. Commentaries, abstracts only, conference proceedings, consensus statements, reviews, case reports, case series, dissertations, and articles on ethical or legal issues.

FQoL = Family quality of life; HRQoL = Health-related quality of life.