Participants and procedures:
This is a cross sectional study. The study data obtained from the Yazd Health Study (YaHS). YaHs is a prospective cohort study started in 2014 to investigate chronic diseases' incidence and prevalence, as well as the factors affecting them in the Yazd Greater Area, located in the center of Iran with 582682 population. The sampling method of the study was multi-stage stratified so that in the first stage 200 clusters were randomly selected from the different areas of Yazd Greater Area in cluding Yazd city, three annexed cities and annexed villages. Then, based on the household postcodes, the heads of the clusters were selected and the questionnaires were completed. Details of YaHS was published elsewhere[28].
Ten thousand people lived in Yazd in the age range of 20 to 70 years participated in this study. Among them, 1901(696 with diabetes and 1205 helthy old people) were elderly 60≥, and the data gathered from them were used to assess the psychometrics of the SF-8 scale. In this study, we tried to select the number of women and men in the same age groups [28]. The inclusion criteria were an age of 60 years and above at the time of the interview, as well as informed consent to participate in the study. The participants unable to answer questions (e.g., due to hearing deficits, aphasia, cognitive impairment, mental disorder) were excluded.
Measurements
Health-related quality of life [SF-8]:
The SF-8 is a short version of an original 36-item health survey or health-related quality of life (SF-36). It has eight domains, including general health (GH), physical functioning (PF), role limitations due to physical problems (RP), bodily pain (BP), vitality (VT), social functioning (SF), mental health (MH), and role limitations due to emotional problems (RE). It is a generic multipurpose short-form quality of life instrument developed by the RAND Corporation and the Medical Outcomes Study (MOS) in the 1980 s[24], with two physical (PCS) and mental (MCS) dimensions. Each item of the SF-8 is assessed using a 5 point Likert scale.
Pittsburgh Sleep Quality Index [PSQI]:
The Pittsburgh Sleep Quality Index (PSQI) is an instrument used to measure the quality and patterns of sleep in older adults. It differentiates "poor" from "good" sleep by measuring seven domains: subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, and sleep disturbances, use of sleep medication, and daytime dysfunction over the last month. The subject self-rates each of these domains. Scoring the answers is based on a 0 to 3 scale, whereby 3 reflects the negative extreme on the Likert Scale [29]. The overall score of this questionnaire is 21, and the overall score of 6 or above indicates that the quality of sleep is poor [29-31]. Reliability of the PSQI was assessed in previus study in terms of internal consistency and corrected item–total correlation. Internal consistency analysis[0.77] showed that persian version of PSQI is acceptable in term of relaiblity and validity[32].
Scale preparation steps
Translation procedure:
Several steps were taken to translate the SF-8 instrument based on international guidelines to assure the accuracy of the translation procedure: (1) forward translation: two bilingual native Iranians with background of social science and gerontology translated SF-8 from English into Persian, independently. (2) Both translators and a project manager compared the translated versions and discussed the controversies to unify the two translated versions. (3) Backward translation: two professional English language translators back translated this Persian version into English.
The translators of the second step were not aware of the original English version of the SF_8 questionnaire. This step was conducted to identify conceptual inconsistencies between the translated and original versions of the questionnaire. (4) Each item and the entire questionnaire were reviewed by the research team, and finally the questionnaire was approved with the consensus of all members. (5) The 8 item of health related quality of life was implemented on 50 old people to understand how they interpret the items on the questionnaire. (6) In the last step: all the necessary changes were applied based on the suggestions obtained from the initial stages as well as the pilot study on the final version of the questionnaire and finally it was implemented on 1901 old people.
Content validity:
The prepared questionnaire was sent to the experts (6 expert) of different disciplines including gerontology, social welfare, epidemiology, and health education for review of content validity. They were asked to comment on the relevancy, clarity and simplicity of the items. To analyze the data regarding the content validity, two indicators including content validity ratio (CVR) and content validity index (CVI) were used.
Assessment of internal consistency, reliability, and stability:
In order to determine the questionnaire's internal consistency, Cronbach’s alpha and to evaluate the questionnaire stability test-retest method were used and intra-class correlation index, also, calculated. Minimum Cronbach’s alpha (0.7) and intra-class correlation index (0.6), were considered as acceptable [17].
The construct validity of the questionnaire
Discriminant validity, Convergent validity, and Confirmatory factor analysis [CFA] were used to determine the structural validity.
Confirmatory factor analysis:
There are various indices to evaluate the model, each of which considers a certain aspect of suitability. The comparative fit index (CFI), incremental fit index (IFI), the root mean square error of estimation (RMSEA), and the goodness of fit index (GFI) all were used to calculate the model fit. If indices IFI, GFI, and CFI were between 0 to 1, and the values obtained were closer to one, the suitability of the model to data was better [33]. RMSEA values between 0.08 to 0.1 showed moderate fitness and lower than 0.08 showed a good fit model [33].
Discriminant validity:
For discriminant validity, the known group was assessed by comparing the SF-8 scores between groups with Student's t-tests, 2‐tailed, P < 0.05.
Convergent validity:
Because many studies have examined the relationship between health related quality of life with mental health and sleep quality but not specially in the elderly, As well as due to inconsistencies in the results of previous studies[34-36]; the Pittsburgh Sleep Quality Index was used to assess convergent validity.
Data analysis:
A significance level of 0.05 was considered, and statistics software SPSS version 24 and AMOS version 24 were used.