Protocol for a Systematic Review Exploring the Psychometric Properties of Health and Wellbeing Measures Used With Adolescents Experiencing Intellectual Disabilities.

Whilst there are studies that have systematically reviewed the psychometric properties of Quality-of-Life measures for children and young people experiencing intellectual disabilities, these narrowly focus on disease or health conditions. The objective of this planned systematic review is therefore to collate, summarise and critically appraise the psychometric properties of health and wellbeing measures used with adolescents (aged 11–16) with an intellectual disability. We designed and registered a study protocol for a systematic review of studies which explores the psychometric properties of health and wellbeing measures used with adolescents experiencing intellectual disabilities. Electronic databases including PsycINFO, CINAHL, MEDLINE and ERIC will be searched using predened search terms to identify relevant studies. Quantitative and mixed-methods studies, and studies published in peer-reviewed journals or grey literature will be included. Review papers, editorials and case studies will be excluded. Eligible studies should identify self and/or proxy reported health and wellbeing measures which assess health and wellbeing among adolescents with intellectual disabilities. The methodological quality of the included studies will be assessed by applying the COSMIN Risk of Bias Checklist. The quality of the evidence (i.e., the total body of evidence used for the overall ratings on each psychometric property of an instrument) will be evaluated in accordance with the GRADE guidelines.

The protocol has been registered at the International Prospective Register of Systematic Reviews (PROSPERO). The registration number is: CRD42021231697.

Background
Wellbeing can be understood as how people feel and function, both on a personal and social level, and how they evaluate their lives as a whole (New Economic Foundation, 2012). The concept of wellbeing has been de ned and operationalised in many ways and tends to encompass both objective (e.g., income, education, and health) and subjective (e.g., happiness, perception of quality of life and life satisfaction) aspects of a person's life (Statham & Chase 2010). According to the World Health Organisation (WHO, 2021), health is a state of complete physical, mental, and social wellbeing, not merely the absence of disease or in rmity. The WHO de nition links health explicitly with wellbeing and conceptualises health as a human right requiring physical and social resources to achieve and maintain. In recent years, the concept of children's wellbeing has become rmly embedded in discourse and policy in the academic, government and public sectors in countries across the world (e.g., Statham & Chase 2010). Since the publication of UNICEF's Report Card 7 (2007) entitled 'Child poverty in perspective: An overview of child wellbeing in rich countries' children's wellbeing has been put rmly on the agenda in many countries around the world (UNICEF, 2007).
Wellbeing is a particularly important concept for children and young people with intellectual disabilities (ID). An ID is a disability characterised by signi cant limitations in both intellectual functioning and in adaptive behaviour, as expressed in conceptual, social, and practical adaptive skills (AAIDD, 2010). It is a generalised disorder, which encompasses a wide array of cognitive and behavioural de ciencies; and affects the individuals in all areas of life (AAIDD, 2010). The severity of ID is classi ed into four types, based on an intelligence quotient (IQ) test, namely mild, moderate, severe, and profound ID (AAIDD 2010).
Children and adolescents with an ID are more likely to experience diminished wellness as they age, as well as having an increasing number of chronic conditions and lower socioeconomic status than their non-disabled peers (Menear et al., 2015).
Whilst there is a wide range of research examining adolescent's health and wellbeing, few of these studies include children and young people with ID (Boström & Broberg, 2018;Davidson et al., 2017). Current large-scale global studies of wellbeing such as the WHO Health Behaviour of School-aged Children (HBSC) and Children's Worlds do not include children who experience ID. This is of particular concern given that individuals with ID tend to experience poorer health, as well as score lower on a number of health and wellbeing indicators in comparison to their typically developed peers (Emerson, 2011). Many health and wellbeing measures are usually administered by survey and are not accessible to youth with ID. This population may have di culty understanding the format and complexity of questions and response scales used in questionnaires designed for same-age peers (Boström et al., 2016;Davidson et al., 2017), as intellectual impairment is often associated with di culties in communication (Shevell, 2008), working memory (Lifshitz et al., 2011) and self-insight (Jahoda et al., 2010), all of which are required to respond effectively to questionnaires. In 2011, Scott and colleagues carried out a pilot study including children (aged 11-17 years) with ID/Special Educational Needs in the WHO HBSC Questionnaire. Using the standard protocol, just over half of the children were able to 'complete' the survey questionnaire, however a high proportion of their responses proved to be uncodable (Scott et al., 2011).
Children and young people with ID have traditionally been assessed via proxy reports completed by parents, teachers, or carers as it is assumed that these young persons do not have the capacity to selfreport on these domains (Shevell, 2008). Thus, a notable limitation is that a third party is reporting on the wellbeing of the person with an ID (Hayes et al., 2013). More broadly, since measuring health and wellbeing involves a subjective perspective, the use of proxy-reports to determine health and wellbeing has been questioned, and it has been concluded that self-reports cannot simply be replaced by proxyreports (Verdugo et al., 2005). It is now appreciated that the young person's view should, where possible, be sought directly rather than being inferred from proxy-reports. However, proxy-reports should not necessarily be discarded, but rather employed as an additional source of information to an individual's health and wellbeing (Clark et  . The WellSEQ is speci cally tailored to measure self-rated mental and ill health, peer relations and school and family environment in adolescents with mild or moderate intellectual and developmental disability and is administered via an interactive touch-based application for tablet PCs (Boström et al., 2016). The standard method for collecting reliable self-reports from persons with ID has consisted of structured interviews (e.g., Emerson, 2005; Haynes et al., 2013), but because this is a highly time-consuming procedure, there has been a shift of focus to design questionnaires speci cally for this population. As a result, in the past decade, and in line with the trend of stimulating participatory research in people with IDs, there seems to be an increase in studies using self-reports to assess the health and wellbeing of adolescents with ID.
Researchers have starting to address the dearth of instruments suitable for measuring health and wellbeing with adolescents in the ID population. Whilst there are studies (e.g., Carlon  adolescents with ID will be identi ed, and recommendations will be gathered to determine their reliability (i.e., to identify what works). This is urgently needed as clinicians, researchers and service providers require information about the methodological quality and the characteristics of measures to make informed decisions about the most reliable and valid tool for a speci c purpose (Kipfer and Pihet, 2019). It is expected that this systematic review will assist in informing choice when selecting an instrument for the measurement of health and wellbeing with this population. It will contribute to the knowledge surrounding available and appropriate measures to use for measuring the health and wellbeing of adolescents with ID, which are necessary to inform intervention development and future health policy.
The aim of this systematic review is to collate, summarise and critically appraise the psychometric properties of health and wellbeing measures used with children and young people (aged 11-16) with ID.
The proposed systematic review will answer the following questions: 1. What measures are being/have been used to assess health and wellbeing of adolescents with ID? 2. What is the methodological quality of these health and wellbeing measures for use with adolescents with ID? 3. What measures work best for assessing health and wellbeing of adolescents with ID?

Methods
Studies included in the systematic review will use the Patient/Problem, Intervention Comparison, Outcome and Time or PICOT (Fineout-Overholt and Johnson, 2005) mnemonics for reviews. PICOT based eligibility and inclusion criteria are described in Table 1. March 2021 and available in the English language will be considered.
Quantitative and mixed-methods studies, and studies published in peer-reviewed journals or grey literature will be included. Grey literature will be searched for additional reports and papers. Using our search terms web searches Google and Google Scholar, and websites of any charitable and nongovernmental organisations will be searched to obtain relevant reports i.e., Barnardo's, Save the Children etc. EThOS (e-thesis online service) will also be used to search for thesis's and dissertations relevant to our study. Review papers, editorials, or case studies will be excluded.
Eligible studies should report instruments designed for use with children or adolescents (aged [11][12][13][14][15][16] with ID to assess their general, subjective and/or objective health and wellbeing, including concepts such as health-related quality of life, life satisfaction, happiness, and wellness. Studies designed for adults or older people with ID, and studies that do not assess health and wellbeing domains will be excluded. Studies from all countries will be included as long as they are available in an English translation. Studies that have not been translated into English will be excluded. The study population will include adolescents between the ages of 11 and 16 years. If a study includes a broader age range encompassing 11-16-year-olds, and if it is possible, data will be extracted for only the targeted age groups. Participants must have an ID diagnosis (mild, moderate, severe, or profound). Young children (aged 10 or younger), young adults (17 years or above) and elderly people (over 70). Studies will be excluded if it is not clear whether participants have an intellectual disability, or where they have other conditions (i.e., autism, epilepsy, or physical disabilities) without speci cally noting that they also have an intellectual disability.

Data Extraction
Data will be extracted as follows.

Literature Search and Study Selection
Titles and/or abstracts of studies retrieved from the database searches and those from additional sources will be screened independently in Covidence by two review authors (SM and JD) to identify studies that meet the inclusion criteria outlined above. The full text of these potentially eligible studies will be retrieved and independently assessed for eligibility by two review authors. Any disagreement between them over the eligibility of studies will be resolved through discussion with a third review author (MM). The interrater agreement will be assessed by calculating weighted k (Cohen & Humphreys, 1968) and interpreted as very good (0.81-1. A standardised, pre-piloted form will be used to extract data from the selected studies for assessment of study quality and evidence synthesis. Two review authors will record information related to: (1) study characteristics (i.e., study setting; study population, study sample (n); study methodology) (2) instrument characteristics (i.e., reporter (self and/or proxy); type of instrument; format of the instrument (i.e., number of items and response formats), psychometric properties of the instrument (e.g., Cronbach's alpha, reliability analysis); and the results of the measure/study). Where needed, we will contact authors for original data with a maximum of three contact emails: once a month over three months.

Study Quality Assessment & Data Synthesis
Evaluation of Methodological Quality of Studies internal consistency (box 4); cross-cultural validity\measurement invariance (box 5); reliability (box 6); measurement error (box 7); criterion validity (box 8); hypotheses testing for construct validity (box 9); and responsiveness (box 10) . Only checklist items that are assessed in each included study will be completed since not all measurement properties are evaluated in all articles. For each study, an overall judgement will be provided on the quality of the particular study. We will use a four-point rating system where each standard within a COSMIN box will be rated as 4 = 'very good', 3 = 'adequate', 2 = 'doubtful' or 1 = 'inadequate' . The total score of methodological quality ratings per psychometric property will be presented as a percentage of the ratings: inadequate (0-25%); doubtful (25.1-50%); adequate (50.1-75%); and very good (75.1-100%). Two review authors will rate the methodological quality independently, and any discrepancies will be resolved by consensus.

Evaluation of Psychometric Properties of Instruments
The result of each single study on a measurement property will be rated against the updated criteria for good measurement properties (Terwee et al., 2007) on which consensus will be achieved by the two review authors. Each result will be rated as either: su cient (above the quality criteria threshold: +); insu cient (below the quality criteria threshold: -); or indeterminate (less robust data that do not meet the quality criteria: ?) using the prede ned criteria for good psychometric properties . A 75% agreement rate will be used , that is for an overall su cient (+) or insu cient (-) rating on a psychometric property, 75% or more of the studies reporting the psychometric property must be su cient (+) or insu cient (-). Otherwise, for an overall inconsistent (±) rating, less than 75% of studies will show the same rating, and for overall indeterminate (?) rating, all studies will be indeterminate (?).
The evidence will be summarised, and the quality of the evidence (i.e., the total body of evidence used for the overall ratings on each psychometric property of an instrument) will be graded as high, moderate, low, or very low using the Grading of Recommendations, Assessment, Development and Evaluations (GRADE) guidelines (see Prinsen et al., 2018).

Selection of Instruments
The selection of instruments and recommendation of suitable instruments for future use will be based on a combining overall rating results of each psychometric property (Step 2) and grading results (Step 3) . Each instrument will be classi ed into three recommendation categories : (A) most suitable (i.e., instruments with high-quality evidence for su cient content validity in any aspects of relevance, comprehensiveness, and comprehensibility, and at least low-quality evidence for su cient internal consistency); (B) promising but need further validation studies (i.e., instruments categorized not in A or C); and (C) not recommendable (i.e., instruments with high quality evidence for an insu cient psychometric property).

Discussion
To the best of our knowledge, this is the rst systematic review exploring the psychometric properties of health and wellbeing measures used with adolescents experiencing intellectual disabilities. By providing evidence-based knowledge about measures being used in health and wellbeing research amongst this population, and more importantly how reliable and valid these measures are, the most suitable for use will be identi ed. This is crucial as clinicians, researchers and service providers need information about the methodological quality and the characteristics of measures to make informed decisions about the most reliable and valid tool for a speci c purpose. The ndings of this study will contribute to the knowledge surrounding available and appropriate measures to use for measuring the health and wellbeing of adolescents with intellectual disabilities, which are necessary to inform intervention development and future health policy.
One limitation of the systematic review process is the exclusion of studies that are not available in the English language, which may mean that articles identifying health and wellbeing measures used with adolescents experiencing intellectual disabilities based in non-English speaking countries and not available in the English language will be excluded.

Declarations
Ethics approval and consent to participate.
Ethical approval is not required for this systematic review as no tests, measurements or experiments were performed on humans as part of this work.

Consent for publication
Not applicable.

Availability of data and materials
The studies included in the review will be available from the corresponding author upon request.

Competing interests
The authors declare no competing interests.

Funding
A Baily Thomas Charitable Fund (TRUST/VC/AC/SG/5662-8764) funded SM's research assistance. The funders did not fund protocol development and are not responsible for the content of the review protocol.
Author's Contributions SM led the conceptualisation of the protocol with input from all authors. All authors have read and approved the nal manuscript.