In order to reach the objectives, we performed a multicentric cross-sectional population-based descriptive study which was carried out through a personal interview to people with intellectual disability with the support of their caregivers and in the company of their relatives and/or legal guardians; but with prevalence of the opinion of IIDs. In the interviews, caregivers and social workers collaborated together with the purpose of improving the understanding of the questions and thus improve the reliability and validity of the answers.
The population of the study are institutionalized IIDs who live in residential homes and attend occupational and/or leisure centres in three Spanish provinces (Salamanca, Zamora and Cáceres), who receive comprehensive health and occupational care in their residence and/or home or only during daytime, from 9 a.m. to 5 p.m. Criteria for inclusion in the study included obtaining informed consent by the institutionalized subjects who would participate in the study, either given by the IIDs themselves or by their relatives, as well as receiving an authorization by the director of the centre in which they are institutionalized (see Additional file 1). The sample was randomly selected and it returned 162 people with a very similar distribution in the three centres: 55, 51 and 46 people, respectively.
Data collection procedures/tools
The measuring instrument used was a structured questionnaire, which was completed during an individual/family interview and which includes, on the one hand, sociodemographic variables which will make it possible to characterize the sample (age, sex, percentage of disability, cause of disability, environment and place of residence); and on the other hand the scales for the assessment of function and quality of life parameters which are being studied: Functional Independence Measure (FIM) scale , Duke-UNC Functional Social Support Questionnaire [9, 10], and the SF-36 Health Survey [11,12,13,14].
The Functional Independence Measure (FIM) scale is a globally accepted tool to measure the degree of disability and the independence of the patients for their activities of daily living (ADL). The scale measures 18 activities which are classified into two dimensions, physical and cognitive, and which are in turn divided into 6 categories which measure self-care, sphincter control, transfer, locomotion, communication and social cognition. Each item is scored from 1 to 7, ranging from total dependence to independence, and the total score ranges from 18 points (total dependence) to 126 points (complete independence), so that the lower the score, the worse the functional level of the patient [1, 2]. In this study, we have chosen the FIM scale, because the variables it measures include cognitive aspects, communication and interaction with the environment, which are key elements in the life of IIDs. The FIM scale is currently a standard in the global literature which is widely used for different pathologies and age groups, and which has proven to be a valid, sensitive and reliable instrument for the functional assessment of disability.
The Duke-UNC Functional Social Support Scale is a simple and brief tool which assesses the perceived functional or qualitative social support, adapted to the Spanish population and widely used in the caregiver community [8, 9]. This scale includes 11 items which are scored on a Likert-like scale ranging from 1-“Much less than I would like” to 5-“As much as I would like”; so that the final score is a quantitative measure of the perceived social support: the lower the score, the less the perceived support. The score range of global functional support moves between 11 and 55 points, and the cut-off score to consider that there is perceived support is < 32 points. The scale is divided into two subscales: confidential social support, which represents the possibility to communicate with other people (items 1, 4, 6, 7, 8, 10); and affective social support, which assesses the received displays of affection and empathy, as well as the availability of people for those displays of affection (items 2, 3, 5, 9, 11) [9, 10]. It has a grade B of recommendation according to experts .
Quality of life was measured with the SF-36 Health Survey, the most widely used questionnaire globally which assesses health-related quality of life in terms of physical and psychological functioning through eight dimensions: Physical Functioning-PF (physical limitations), Role Physical-RP (interference with work and daily activities), Bodily Pain-BP (intensity of pain and its effect on activities), General Health-GH (personal assessment of health), Vitality-VT (feeling of energy), Social Functioning-SF (interference with normal social activities), Role Emotional-RE (interference with work or other daily activities) and Mental Health-MH (depression, anxiety, control of emotions and behaviour). It is expressed as a score on a 0–100 scale in which higher scores represent better quality of life. This questionnaire has a grade A of recommendation according to experts  and has been used with people with low levels of education and IIDs [16,17,18]. HRQoL with social supports improves individual functioning and, consequently, improves both physical and emotional Well-being.
The fieldwork was carried out during the last quarter of the year 2015 and the first quarter of the year 2016. The interviews were made in the centers where IIDs are institutionalized and always in the presence of the caregivers and social workers.
The personal interview as a social research technique makes it possible to obtain the desired information from a specific subject beforehand through a direct conversation, based on a structured questionnaire. That is, all the subjects interviewed are asked the same questions, in the same way, and in the same order. Therefore, the questionnaire was applied to the selected IIDs in front of their families or of professionals from supporting institutions in cases in which the subject had a greater dependence. In all cases, questions were adapted to the level of understanding of IIDs by social workers. They have great experience in dealing with IIDs. In this way, we try to control bias and improve the validity of the study.
This study was reviewed and approved by the Clinical Research Ethics Committee for Clinical Investigation of the University Hospital of Salamanca (see Additional file 2).
The data obtained were included in a database which had been specifically created for this research, and they were analysed with the statistical software package SPSS 23.0. The statistical analysis included a descriptive study of frequency distribution of all variables (univariate analysis). Quantitative results are expressed as arithmetic mean and standard deviation (SD), qualitative results are expressed as relative frequencies (percentage or proportion), accompanied by their corresponding 95% confidence intervals (95% CI), which make it possible to assess the population parameter through the values observed in the different variables of the individuals in the sample. On the other hand, a multiple linear regression model was used to explore and quantify the relation between the scores (quantitative variables) obtained in the different scales/questionnaires of functional assessment and quality of life that were applied (multivariate analysis). The results are included in simplified tables which show the regression coefficient (B), 95% CI for B and statistical significance (p). Finally, the researchers in this study have defined a p-value for statistical significance of p < 0.05.