Reliability and validity
DISEASE
The tool’s validity was checked by face, content and construct validity. To determine face validity, the questionnaire was completed by 20 students. Content validity of the questionnaire was confirmed by qualitative and quantitative methods. In quantitative content analysis, Content Validity Ratio (CVR) and Content Validity Index (CVI) were evaluated by 11 experts. Then, questions with CVR of 0.59 and CVI of 0.79 and higher were preserved in the questionnaire. In order to evaluate the tool’s reliability, Cronbach’s alpha was calculated. Cronbach’s alpha of between 70 and 80% was set to be adequate and acceptable for internal consistency [Test–retest was employed to investigate the stability of the instrument over time. The questionnaire was completed by 40 adolescents with an interval of 2 weeks. A correlation coefficient of higher than 0.7 was considered acceptable for internal correlation coefficient (ICC). The following formulas were used to evaluate the content validity ratio (CVR) and content validity index (CVI):Finally, the designed questionnaire had two parts:Part one: Demographic questions about fathers’ job, mothers job, fathers’ education level, mothers’ education level and self-report economic situation.Part two: Constructs of the knowledge (general knowledge, disease symptoms, behavioral risk factors, mid-term outcomes and long-term outcomes), health beliefs (perceived self-efficacy, behavioral beliefs, perceived susceptibility, and perceived severity) and behavior (prevention of stress, healthy food/healthy diet, unhealthy food/unhealthy diet, high-risk behavior, and self-care) (Table Description of study instrument1–2 unit = 1, 2–3 unit = 2, 3–4 unit = 3, 5–4 unit = 4, I do not consume at all = 0 (0–8)1 unit = 1, 2 units = 2, 3 units = 3, I do not consume at all = 0 (0–4)3 items (Every day of the weekto I don't eat at all)Less than 1 h = 4, 1–2 h = 3, 2–3 h = 2, more than 3 h = 1, I do not use TV, tablet, mobile phone or electronic games at all = 5 (1–5)Always = 4, often = 3, sometimes = 2, rarely = 1, never = 0 (0–8)One vitamin D supplement a week = 4, one every 2 to 3 weeks = 3, one a month = 2, one a year = 1, I don't take it at all = 0 (0–4)Once a month = 4, once every 2–3 months = 3, once every 6 months = 2, once a year = 1, I don't measure at all = 0 (0–4)Less than 7 h = 2, 7–8 h = 3, more than 8 h = 1 (1–3)