The Effectiveness of Rehabilitation Interventions on the Employment and Functioning of People with Intellectual Disabilities: A Systematic Review

Purpose This systematic review analyzed the effectiveness of rehabilitation interventions on the employment and functioning of people with intellectual disabilities (ID), as well as barriers and facilitators of employment. Methods This was a systematic review of quantitative, qualitative, and mixed methods studies. The outcomes were employment, transition to the open labor market and functioning. The review included qualitative studies of employment barriers and facilitators. The population comprised people with ID aged 16–68 years. Peer-reviewed articles published in English between January 1990 and February 2019 were obtained from the databases Cinahl, the Cochrane Library, Embase, Eric, Medic, Medline, OTseeker, Pedro, PsycInfo, PubMed, Socindex, and the Web of Science. We also searched Google Scholar and Base. The modified selection instrument (PIOS: participants, intervention, outcome, and study design) used in the selection of the articles depended on the selection criteria. Results Ten quantitative (one randomized controlled, one concurrently controlled, and eight cohort studies), six qualitative studies, one multimethod study, and 21 case studies met the inclusion criteria. The quantitative studies showed that secondary education increases employment among people with ID when it includes work experience and personal support services. Supported employment also increased employment in the open labor market, which sheltered work did not. The barriers to employment were the use of sheltered work, discrimination in vocational experience, the use of class teaching, and deficient work experience while still at school. The facilitators of employment were one’s own activity, the support of one’s family, job coaching, a well-designed work environment, appreciation of one’s work, support form one’s employer and work organization, knowledge and experience of employment during secondary education, and for entrepreneurs, the use of a support person. Conclusions The employment of people with ID can be improved through secondary education including proper teaching methods and personal support services, the use of supported work, workplace accommodations and support from one’s family and employer. These results can be utilized in the development of rehabilitation, education, and the employment of people with ID, to allow them the opportunity to work in the open labor market and participate in society. Electronic supplementary material The online version of this article (10.1007/s10926-019-09837-2) contains supplementary material, which is available to authorized users.

The prevalence of ID is about 1% of the population, but it differs between countries [12]. In Finland, it is 1% [13,14], as in most European countries [15]. In Finland, 0.8% of working-aged people have ID, which means 25,000 people [6].
Rehabilitation is a goal-oriented process, which here aims to enable people with ID to reach an optimum mental, physical and social functional level, thereby providing them with the tools required to change their lives [16]. People with ID should receive services that support their functioning, self-determination and participation [2,17]. In the last years, instead of using a system-centered approach, services for people with ID and other disabilities have begun to use a person-centered approach, tailoring services around the individual rather than enforcing a one-size-fits-all principle [2,18,19]. According to Austin and Lee [20], job-related services (job search assistance, job placement, job readiness training, on-the-job support services) significantly predicted the employment outcomes of people with ID. However, they found no significant relationship between personal services (diagnosis and treatment, counseling and guidance, transportation, maintenance, miscellaneous training) and employment outcomes.
Sheltered work or workshops are one form of rehabilitation for people with ID. One of the main tenets often cited by supporters of sheltered workshops is that center-based programs provide employment opportunities for people with ID in a segregated environment by building up their vocational and social skills [21,22]. However, young adults with mild to moderate ID also have difficulties transitioning from sheltered workshops to the open labor market. Moreover, parents' overprotection, important friendships, and the attitudes of employers and co-workers can be barriers during this transition process [22].
In this review, the term intellectual disability (ID) refers to people who have a significantly reduced ability to understand new or complex information and to learn new skills (impaired intelligence), and who have reduced ability to cope independently (restricted social functioning). In the United Kingdom (UK) the term 'people with learning disabilities' (LD) is used to describe those referred to elsewhere as people with 'intellectual disabilities' or 'developmental disabilities' [23].
No reviews were found on the effects of rehabilitation on employment among people with ID. Some reviews, however, evaluated the effectiveness of rehabilitation on the functioning (activities of daily living, self-care skills, communication skills, and cognitive achievements) of people with ID [16], the effects of the person-centered planning of services for people with ID on different outcomes (i.e., employment) [17], and effects of technology use on employment [24]. The research questions were: 1) How effective are rehabilitation interventions for the employment of people with ID, 2) what are the barriers to and facilitators of employment of people with ID, and 3) what kind of individual support measures have been used to increase the work ability and functioning of people with ID?
The theoretical orientation guiding this study was the International Classification of Functioning, Disability and Health (ICF) framework [25] (Fig. 1). The ICF identifies and classifies the domain of environmental factors, including rehabilitation, as one of its health-related domains [26][27][28]. ICF provides a detailed framework for describing disease experiences as a dynamic interaction between its components, and 'core sets', comprising lists of ICF categories [28]. The ICF identifies and classifies the component of environmental factors, including rehabilitation, as one of its health-related components [26,27]. According to the ICF, these environmental factors can also be either barriers to or facilitators of employment for these individual [27].

Methods
This systematic review analyzed the effectiveness of rehabilitation interventions on the employment and functioning of people with ID, as well as barriers and facilitators of employment. We included quantitative, qualitative, and multimethod studies, because they provided different knowledge regarding this phenomenon [29,30]. Quantitative studies showed the effectiveness of rehabilitation; qualitative studies showed the process, facilitators and barriers; and multimethod studies combined these.
We used the modified selection instrument (PIOS: participants, intervention, outcome, and study design) [29,31] to select the titles, abstracts, and full papers according to the selection criteria. The inclusion criteria for studies were randomized controlled trials (RCTs) and non-randomized studies such as concurrently controlled trials (CCTs), case-control studies, cohort studies, follow-up studies, and case studies that investigated the effectiveness of rehabilitation among people with ID. We expected the outcomes of the studies to be employment (supported or non-supported employment, transition from school to work, transition from sheltered workshop to employment), or functioning (job performance), and the population to comprise people with ID (developmental disabilities, learning disabilities, cognitive disabilities, mental retardation, mental handicaps), in the age group of 16-68 years.
We searched for articles published in English from January 1990 to February 2019. The databases we searched in February and October in 2016 and in February in 2019 included: Cinahl, the Cochrane Library, Embase, Eric, Medic, Medline, OTseeker, Pedro, PsycInfo, PubMed, Socindex and the Web of Science. In addition, we searched Google Scholar and Base. Our main search terms were: (1) intellectual disability, mental disability, mental retardation, mentally disabled persons; (2) employment, employability, sheltered work, work capacity, work ability, vocational status, career; (3) rehabilitation, treatment, therapy, training, career counseling, social support, education, program, intervention, assistive technology, counseling, mainstreaming; (4) outcomes, outcome assessment, randomized controlled trials, program evaluation, effectiveness, validation studies, evaluation studies, comparative studies, cost-benefit analysis, before-after and follow up. We also manually scanned reference lists of identified reviews for additional relevant articles. Full details of the search strategy are available (Supplementary material).
The review team comprised five researchers, whose areas of expertise were disability, ergonomics, rehabilitation, social sciences, interventions, systematic reviews, and quantitative and qualitative methodology. First, pairs of researchers independently reviewed titles and abstracts and made a unanimous decision. If consensus was not reached, a third researcher was consulted. The full texts of all the eligible articles and those whose eligibility could not be discerned from reading the abstract were obtained.
Two reviewers independently assessed the methodological quality of the included RCT and CCT studies using the checklist by van Tulder et al. [32], which has also been used in other reviews concerning rehabilitation [28,33,34] and workplace accommodation [35]. The checklist consisted of 11 criteria. Each item scored two points for 'Yes', one point for 'Don't know', and 0 points for 'No'. The item scores were summed into a single total score (range 0-22). Studies were considered to be of high methodological quality if the score was at least 11 out of 22, and of low methodological quality if they achieved fewer than 11 points [32].
The cohort studies and the mixed method study were assessed using the Newcastle-Ottawa Scale (NOS) [36]. This assessment scale consists of eight items within the following three categories: selection of study groups (four items), comparability of groups (one item/two questions) and ascertainment of exposure/outcome (three items). The highest value for quality assessment is nine 'stars'. One star is allocated for each item in the selection and outcome categories and two stars in the comparability category. Study quality was graded as follows: 1-3 stars (low quality), 4-6 stars (intermediate quality), and 7-9 stars (high quality) [36].
Three pairs of reviewers independently assessed the methodological quality of the included qualitative studies using a modified version of the CASP method [37]. Any difference in the item scoring was resolved through discussion with the third author until consensus was achieved. This assessment tool consisted of evaluation criteria that are commonly acknowledged as crucial in qualitative research in the

Fig. 1
The theoretical orientation that guided this study was the ICF (International Classification of Functioning, Disability and Health) framework [25] 1 3 social sciences. The assessment tool involved 10 questions based on the following four main principles: The research should (1) provide a defensible research strategy that can answer the questions posed, (2) demonstrate rigor through systematic and transparent data collection, analysis and interpretation, (3) contribute to advancing wider knowledge and understanding, and (4) demonstrate credibility with plausible arguments about the significance of the findings. Each item scored 'Yes' or 'No, depending on whether the topic was described sufficiently. An additional score of 'Partially' was added, as the original checklist was not able to adequately differentiate between the quality of the studies [38]. This addition resulted in three options: 'Yes' (2 points), 'Partially' (1 point), and 'No' (0 points). The higher the total score, the better the methodological quality, with a maximum score of 20. The studies were rated as being of high methodological quality if they achieved more than 10 points.
The case studies were assessed using eight questions from the Joanna Briggs Institute (JBI) method [39]: (1) Were the demographic characteristics of the person attending clearly described? (2) Was the history of the person attending clearly described and presented in the timeline? (3) Was the current clinical condition of the person clearly described, (4) Were diagnostic tests or assessment methods and the results clearly described? (5) Was the intervention(s) or treatment procedure(s) clearly presented? (6) Was the post-intervention clinical condition clearly described? (7) Were adverse events (harms) or unanticipated events clearly described? (8) Does the case report provide takeaway lessons? Each question scored one point for 'Yes', two points for 'No', three points for 'Unclear', and four points for 'Not applicable'.
All the reviewers participated in the data extraction, which was carried out separately to the quantitative, qualitative, and multimethod studies, and included details on the participants and descriptions of the rehabilitation and the outcomes. The meaningful concepts of each outcome were linked to the ICF categories [40,41].
We descriptively synthesized the data using tables to describe the characteristics, results and quality of the included studies. Different tables were used to describe the quantitative, qualitative and case studies. We categorized the overall quality of the quantitative studies and their outcomes using the principles of GRADE (Grading of Recommendations, Assessment, Development, and Evaluation) [42]. The GRADE approach classifies the quality of evidence as high, moderate, low or very low. Randomized controlled studies' evidence begins with strong and cohort studies with scant evidence on the basis of the assumption that randomization is the best method for controlling unknown factors. Five factors can weaken evidence: (1) the risk of bias, which results from the weakness of the research method and its implementation, (2) differences and inconsistencies between different studies, (3) indirectness of the results in comparison to the PICO criteria, (4) the inaccuracy of the results (the number of events and participants and their effect on confidence intervals) and (5) publication bias (Chapter 12, Cochrane Handbook). Thus, strong research evidence required at least two-high quality studies, the results of which were parallel. Moderate research results required one or several high-quality studies, the results of which were only slightly contradictory, or several adequate studies, the results of which were parallel. Scant research evidence meant that the research results had significant contradictions. No research evidence meant that no studies existed or that they were methodically weak.

Results
The search strategy identified 2618 references (Fig. 2). After removing duplicates, 1848 references remained. Two of the reviewer (altogether five) authors scrutinized all the titles and abstracts according to the inclusion criteria, and when information necessary for inclusion was lacking, they read the articles' full texts. We obtained the full texts of 244 articles, and 38 studies met our inclusion criteria.
The age of the participants varied between 16 and 51. However, two qualitative studies [53,54] and two case studies [62,64] did not report the age of the participants. The proportion of men and women differed in the studies. In the quantitative studies, the proportion of women varied from 29% [50] to 58% [47]. In one qualitative study [56] all the participants were men and in another qualitative study [54] all the participants were women. Two studies [51,59] did not report the gender of the participants. Seven studies reported the ethnicity of the participants [45, 47-49, 51, 52, 79].
Sixteen of the 38 studies reported the degree (mild, moderate, severe, profound) of ID as a background factor of the participants. People with mild ID participated in two cohort studies [48,51], one qualitative study [57], and one case study [60]. People with either mild or moderate ID participated in three case studies [71,75,76], and people with moderate ID participated in two case studies [68,80]. Renzaglia et al. [65], Simmons and Flexer [66], McGlashing-Johnson et al. [70], and West and Patton [73] focused on people with moderate or severe disabilities. People with severe ID were participants in three case studies [61,67,69]. Two studies [45,77] reported the participants 'level of functional capacity.

Interventions
All studies contained various mixtures of intervention components. The interventions were carried out during secondary education, the transition from education to work, jobseeking and sheltered work. In general, the content of the intervention was briefly described. However, the stages of the intervention process were poorly reported, and the term rehabilitation was only seldom used.
During secondary education (Upper Secondary School for Pupils with ID) (ICF code d825), the students participated in different educational programs such as national programs, specially designed programs, vocational training and training activities or graduated with inadequate grades [50]. National programs focused on different parts of the labor market, for example, vehicles and transportation, hotels and restaurants, or social and healthcare. Some specially designed programs or individually tailored education were also on offer. The educational programs aimed to improve vocational qualifications and work awareness. The vocational qualification courses included assisting smaller pupils during classes, helping the janitor and working in the school kitchen. Work awareness training courses included watching videos about work, talking about presenting oneself at work, and health and safety instructions at school [46]. Some interventions included postschool and transition services (ICF code e5900) during education to improve the   75.9% of the students reported they engaged in employment sometime after leaving school.
62.1% were currently employed and 42.6% were engaged in full-time employment. Participation in one additional transition activity represented within the employment activities summation variable resulted in students being 1.2 times likely to be currently employed post school. Students were 5.704 times as likely to ever engage in employment after school if they engaged in paid-employment experiences while in school, and 3.489 times as likely to ever engage in employment after school if they experienced a school-sponsored job. However, vocational education was not found to be significantly related to postschool employment features total workplace immersion, and facilitates a seamless combination of classroom instruction, career exploration, and on-the-job training and support.
The goal was the transition of individuals out of sheltered workshops to competitive employment Eight persons completed the course and five of them were employed. The program emphasized learning about other job opportunities in the community (e.g., touring local businesses). Individuals with significant disabilities can imagine themselves working in the community when they are exposed to a variety of career opportunities and can directly see and under-stand what the job entails The cohort model of Project SEARCH allowed ten individuals to exit the workshop and transition into a community setting with familiar peers. This alleviated some of the initial fears of losing friends as a result of leaving the workshop. During the course the participants became more independent and lived in the community, and their views about the meanings of employment changed. Participants demonstrated an increased awareness of employment as a means to take greater responsibility as a productive member of their house-hold and the community. Competitive employment represented the means to earn wages in order to pay their own bills and not have to rely on Medicaid benefits The sample size is small and localized. While the results of this study provide some level of insight regarding the experiences of individuals with IDD who are transitioning from a sheltered workshop into competitive, integrated employment, the views expressed by program participants are limited to their own unique experiences The interviewed women initially went from compulsory school to a vocational preparation measure or directly into a sheltered workshop or occupational therap. Main category: 'coming to terms with disability through vocational participation' was formed on the basis of two key categories: discrimination at school due to learning difficulties and post-school orientation phase as a process of self-determination  The work-related performances of all four employees improved after the three phases of the Self-Determined Career Model. Positive changes were evident between baseline and intervention conditions and continued in the maintenance phase.   Results/Outcomes: All three employees (participants) were able to complete tasks (a planting sequence in a community greenhouse) with no significant problem behavior following multicomponent intervention, and were able to work four hours at a time. In addition, the greenhouse managers reported considerable confidence in the job coach's ability to deal with any behavior difficulties, keeping both coworkers and property safe from harm; reported that problem behavior was almost never severe following the intervention, and acknowledged the employees as productive members of the greenhouse team  carer in a university vivarium (competitive employment) A five-stage process included job task analysis, pre-baseline assessment, baseline assessment, treatment, and post-treatment follow-up. 1) Task analysis involved direct observation of co-workers performing the job tasks, performance of the tasks by the principle investigator (i.e., job coach), and reviewing the recorded task sequences with Phil's supervisor.
2) The second state was to evaluate Phil's current performance and to identify the specific skill areas in which training and retraining was needed.
3) The baseline data on the nine skills were collected for a five-day period. 4) Instruction in the nine skill areas identified as deficient in baseline assessment. Systematic instruction procedure using a least-to-most intrusive prompt system. 5) Post-training assessment of the subject's performance under the same non-training conditions as those in the baseline assessment The percentage of job tasks performed correctly in a nineitem task analysis  Performance A statistically significant difference was found between baseline and both auditory prompting systems in both vocational settings. The number of independent task changes ranged at baseline from 0 to 3, during intervention from 2 to 6 with the single-word and from 3 to 6 with the multiple-word prompting system. Performance duration differences between prompting systems were significant for Student 3 (T = 1.00, p < .05, N = 7) and Student 5 (T = 0.00, p < . 05, n = 7). They transitioned through tasks in significantly less time using the multiple-word auditory prompts in the pet store Van Laarhoven et al. [80], USA Both participants increased their vocational skills using mobile devices. There was not a large difference in correct responding when devices were compared, but participants performed slightly better when using their preferred device. The participants selected video prompts more often than other media prompts (i.e. picture prompts) during initial sessions, except Nick during the first session. The participants effectively self-selected and self-faded their reliance on media-prompts as they became more independent with the task West  The income from the business was low, so other sources of income were needed. The support people did not have enough expertise or interest in business. It was difficult to obtain support for running a business, and to make social contacts The support person was present at the workplace and supervised the tasks, and also helped the employee modify working conditions. The amount of support given by the support person gradually diminished, and when the employee succeeded in their tasks independently, the support ended. The maximum duration of support was six months Prior to intervention: Anna worked as a wage-subsidized cleaner for two years. When she moved to another city the wagesubsidy ended. A year later, after her maternity leave, Anna enrolled as a job-seeker, and temporarily attended a sheltered workshop until a job was found for her. However, she did not find employment. The employees felt that Anna could not be employed because of her shyness, lack of initiative and low stress tolerance Adam was registered as a job-seeker in the employment office After the intervention Anna was employed with a wage subsidy in the hotel and restaurant industry. Her job tasks were tailored according to her skills. Later Anna returned to being a customer of the employment office. The support person helped her and again she obtained a wage-subsided job Adam: The support person helped him while he was in his last year of school for people with ID. After school, Adam worked as a trainee, but this did not lead to employment. The support person arranged another job for him, but this did not last very long. Following discussions with the support person, Adam again obtained a place as a trainee, which later turned into a wage-subsided post The factors enabling employment were sufficient duration of support, individually tailored tasks and wage subsidies transition from education to work [43,48,49,51,55,75]. Postschool services included postsecondary education institution accommodations and services, job training services, and life skills services [51]. Employment-related transition services were, for example, vocational assessment, career counseling, pre-vocational education, career-related technical or vocational education, pre-vocational or job readiness training, instructions for job-seeking, job shadowing, job coaching, special job skills training, placement support, internship or apprenticeship programs, work experience at school and other paid work experience [44,48,55,65]. Job tasting was a short, unpaid, time-limited work experience period at the workplace which allowed people to sample a variety of different jobs and work cultures [44]. One qualitative study [57] analyzed the different actors' experiences (schools, agencies, parents, other people) of the transition process and of employment (e310), and Fasching's [54] study analyzed the experiences of people with ID of the transition from school to vocational training and employment. The interventions also included supported work (supported employment, SE) (d855) [66], the use of a job coach or other support person (e340, e360) [45,63], and designing personal solutions (e5900) [64]. One cohort study [46] analyzed whether attending a sheltered workshop (e5900) improved the employment outcomes of supported employees with ID.
Some interventions developed the independence of people with ID, applying the ICF model to the areas of body structures/functions, activities and participation. The interventions included support in psychological and behavioral issues (behavior) (b122) [56,64] and communication abilities (d3) [67]; and a self-regulated problem-solving process in which students set their goals, developed and implemented their action plans (b164, d175) [70], made career-related decisions (d177) [75], were able to perform daily activities (d620, d630) [77], learned navigation skills such as finding the workplace and using public transportation (d4602) [78], and gradually reduced help in carrying out work tasks (d850) [72,75,77].
The use of digital solutions (e135) was intended to improve the daily work performance of people with ID in the open labor market. They could use these tools for receiving digital instructions for work tasks, work processes, work techniques or schedules. These solutions consisted of video modeling and audio coaching [72,76], photo activity schedule books [71], smart phones [74], palmtop-based job aids [69], and self-operated auditory prompting systems [68,80]. In the case studies, the interventions were carried out at workplaces such as restaurants [60,66], hospitals [64], factories and warehouses [76], at markets [79], at schools [70,72], and in conference rooms [80].

Study quality
The RCT study [43] was considered to be of high methodological quality, with scores of 11 out of 22, and the CCT study of Kilsby and Beyer [44] was of low methodological quality, with scores of 9 out of 22 according to van Tulder et al. [32]. The methodological quality of four cohort studies [47,49,50,52] out of six were high, with the maximum of nine 'stars' according to Wells et al. [36]. Four cohort studies [45,46,48,51] and one multimethod study [59] were considered to be of intermediate quality. All six qualitative studies [53][54][55][56][57][58] were considered to be of high quality, with scores ranging from 12 to 20 out of 20, according to the modified CASP method. (Table 6-9, Supplementary Files).

Effectiveness
The quantitative studies showed that supported work increases the employment of people with ID in the open labor market (Table 5). This result was based on one highquality RCT study [43], one high-quality cohort study [47], and one moderate-quality cohort study [45] of altogether 16,947 people with ID. The quantitative studies also showed that both secondary and postsecondary education, including support services and work training, increased the transition of people with ID from school to the open labor market. The result was based on two high-quality cohort studies [50,52] and three moderate-quality cohort studies [46,48,51] with 2,07,484 participants altogether. However, on the basis of one high-quality RCT study [43] and one high-quality cohort study [47] covering a total of 15,089 participants, sheltered work did not increase the employment of people with ID in the open labor market (Table 5).

Barriers to and facilitators of employment
The qualitative studies concerned both the barriers to and the facilitators of employment in the open labor market ( Table 2). The main barriers to employment were that the school and service system tried to guide people with ID towards traditional services such as sheltered work, sometimes against their own needs and interests [57]. However, these people's work skills were not developed or their needs for support were not noticed in sheltered work [58]. Further barriers were discrimination in vocational experience after leaving school [54], poor experiences of class teaching and lack of work experience [58,59].
The main facilitators of employment were people's own activity and support from their families (e310) [57], effective job coaching (e360) [58], a well-designed work environment (e2) [61], appreciation of their work and support from their employer and work organization [56]. Other facilitators were the knowledge and experience of work during education [59], and for entrepreneurs, the use of a support person [53].

Work performance
The case studies showed that the use of digital solutions (e135) improved the work performance of people with ID in the open labor market. Work performance was measured as the percentage and rate of the tasks completed correctly [69,72], the number of movements measured by a costume [76], the number of independent task changes [71], task steps [74], and percentage of intervals with task engagement and interactions [79].

Discussion
People with ID still face specific difficulties in gaining employment, which leads to inequality and social exclusion. Personally-tailored support services, occupational education, work experience and digital solutions can be used to help these individuals become employed in the open labor market.
This systematic review covered 38 studies (one RCT, one CCT, eight cohort studies, six qualitative studies, one multimethod study, and 21 case studies) that investigated the effectiveness of, barriers to or facilitators of employment among people with ID. The main outcome was employment in the open labor market through either supported or nonsupported work, transition from education or from sheltered work to the open labor market, or functioning at work.
The quantitative studies showed that secondary and postsecondary education increases the employment of people with ID in the open labor market when it includes work experience and personal support services. Supported employment also increases employment in the open labor market, but sheltered work does not. The results were in line with those of earlier studies [2,22,81] which have shown the support of a job coach to be important for people with ID to find work, start work and continue at work among.
The qualitative studies showed the barriers to and facilitators of employment in the open labor market among people with ID. The barriers to employment were that the school and service system tried to guide people with ID towards traditional services such as sheltered work; that these people High quality RCT study [43] Employment to supported work after sheltered work 59.6% Employment to supported work without sheltered work 60.4%

9,808
Low High quality cohort study [47] Employment after postsecondary education (OR 2.0) Employment without postsecondary education

11,280
Low High quality cohort study [52] Support of work coach 6.1% No control group 7,090 Very low Moderate quality cohort study [45] Transfer Very low Moderate intervention study [44] were discriminated against at school and had negative experiences of class teaching; and that they did not obtain work experience during education. These results support the findings of earlier studies [6,22] which have shown low rates of employment when individuals come from sheltered work to the open labor market. The facilitators of employment were the people's own activity and support from their families, job coaches, work environments with necessary accommodations, appreciation of their work, support from an employer and work organization, knowledge and experience of work during education; and for entrepreneurs, the use of a support person. These findings support earlier results that have shown the importance of job coaches, employers' responsibility [82] and personally designed support services [17,18,20].
The case studies showed that the work performance of people with ID could be improved through the use of digital solutions in daily work. However, digital solutions are only rarely used among people with ID. One reason may be the employees' low competence in using digital systems, especially in sheltered work. Services in general are becoming increasingly digital, and this also requires more competence from people with ID [83].

Methodological discussion of included studies
Better reporting of basic methodological quality issues is generally required. A better description of participants, such as their gender, age, education, occupation and work experience is also needed when they are people with ID. We can conclude that people with ID are not seen as professionals because, although the studies described their diagnosis or disability well, they did not emphasize their competence and strengths, educational background or work experience. This is also true of studies of disability groups [35].
The studies included several different concepts of disability, rehabilitation, education, performance, and service systems. The concept of ID was defined as 'developmental disability', 'mental retardation', and 'mental disability'. In the UK, 'learning disability' is used in the same way as ID, but in other countries, 'learning disability' means difficulties in learning without ID.
The articles failed to clearly report some elements of the interventions. They should have reported more information about the process and implementation schedule of rehabilitation, the initiator and the place. These shortfalls were found especially in cohort studies that focused on results and background factors such as the participants' diagnoses.
Only few randomized controlled interventions have been carried out to enhance the employment among people with ID. This is possibly because of the low employment rate of people with ID, the low number of implemented rehabilitation programs, the ethical aspects of the study designs, negative attitudes, and a lack of financing instruments for such studies.
The outcomes in this review were employment in the open labor market in either supported or non-supported work, the transition from education or from sheltered work to the open labor market, or functioning at work. With the ICF model as a framework, these outcomes belong to 'participation'. According to the ICF model, rehabilitation belongs to the 'environmental factors' that affect 'activity' (e.g., functioning) as do most of the synthesized themes from the analysis of qualitative studies. Obviously, the primary aim of rehabilitation on a personal level is to enhance the functioning and work ability of people with ID and to enable them to work in the open labor market. On the societal level, it is important to develop and implement solutions that enhance employment and participation and are simultaneously cost effective [17].

Strengths and limitations of this review
The strengths of this review included its multi-scientific research group, the comprehensiveness of the searches, the use of the ICF model as the theoretical framework, and the inclusion of a wide range of studies. The reviewers were experts in different scientific areas, including both quantitative and qualitative methodology and systematic reviews. Every effort was made to insure a comprehensive search. It is possible, however, that we did not find all the relevant studies. Another limitation was that the included studies used different concepts of ID. We included quantitative, qualitative, multimethod and case studies, which showed different kinds of knowledge regarding the process and the effectiveness of rehabilitation. However, this was also a shortcoming of this study because a meta-analyses of several types of results was complicated.

Quality assessment
This systematic review was of 38 studies (one RCT, one CCT, eight cohort studies, six qualitative studies, one multimethod study, and 21 case studies). The quality of the RCT study was assessed using the validated method of van Tulder et al. [32], which has also been used in several other reviews. The quality of the CCT study was low, mainly due to the fact that the study design did not include randomization, treatment allocation, blinding, or intention to treat the analysis. The quality of the cohort studies and the mixed method study were assessed using the Newcastle-Ottawa Scale [36]. The validity and reliability of this method was only partly evaluated in that the content validity and inter-rater reliability were established, but the criterion validity and intra-rater reliability were still in progress [36].
The original qualitative assessment tool CASP was not perceived as very powerful in differentiating between high-and low-quality studies. It only measured whether certain basic items that are essential identifiers of highquality research were mentioned in the report. This type of measurement is crude and makes the scale difficult to use when some of the criteria are implicit in the study. Furthermore, a 'Yes or No' scale does not capture the fact that certain items in the CASP criteria may be more crucial to the quality of the study than others. Adding a third level of assessment, 'Partially', to the method, may solve the first problem. However, the second problem remains: Of the three problematic points of the qualitative studies evaluated, researcher effect is a self-evident fact connected to any study of social life, and is thus less informative than reporting the contribution of a particular study to existing knowledge. Although CASP offers a good basis for evaluating qualitative research reports, it can be further developed by giving different weights to different criteria.
The comparison of the results of the assessment of the quantitative and qualitative studies revealed a bias in that the quality assessment of qualitative studies resulted in several high-quality studies, whereas the quantitative assessment yielded only a few. This was, of course, partly due to different evaluation methods, but it may also be an indication of the different nature of these two types of research. Qualitative studies report interesting new observations about the ways in which participants observe, understand, or experience the phenomenon studied, while quantitative studies aim to make generalizations about possible causes and effects, and reveal other connections between the variables describing the phenomenon being studied. As the knowledge gained through qualitative research is descriptive and not numerical by nature, ranking studies is also difficult.

Conclusions
More people with ID could be employed through personally tailored services and measures. Tailoring can mean secondary or postsecondary education, including proper teaching methods and personal support services, the use of supported work, workplace accommodations, and the support of one's family and employer. Our results can be utilized in the development of rehabilitation, education and employment of people with ID, to provide them with opportunities to work in the open labor market and to participate in society.