Study design and population
This study is part of a 3-year health promotion project comparing the differences in health-related behavior between overweight and non-overweight adolescents in Tao-Yuan County, Taiwan. A cross-sectional descriptive design was used. There are 51 public junior-high and high schools in Tao-Yuan County. To enroll a representative study group, participants were recruited from seven selected schools located in rural (three schools) and urban (four schools) areas that were, respectively, more than 30 km and less than 3 km distant from downtown Tao-Yuan city.
To balance the sample size and gender distribution between overweight and non-overweight samples in the original project, the researchers evaluated BMI data from the school health center before sending the invitation letter. Study subjects comprised nearly half overweight and half non-overweight students aged 13–18 years in grades 7–11 during the first trimester of the school year. They were selected according to the following inclusion criteria: (a) body mass index (BMI), recorded at the respective school health center, was in the rage of 15–30; (b) both the student and the student's guardian provided written consent; (c) there was no physical or mental limitation.
Human subject protection and data collection procedures
The study was approved by the Institutional Review Board (CGMH, No 2004–6145). Informed written consent was obtained from the subjects and their guardian(s) after permission had been obtained from the relevant school administrators. Parents were fully informed about the survey and had an opportunity to review the questionnaire; that is, an invitation letter, emphasizing that the responses would be confidential, were sent together with the questionnaire to each student's guardian. The investigators explained the study to all the students. Students completed the questionnaire anonymously and used about 30 minutes to fill out the scale. They could decline to participate in the project at any time while completing the questionnaire.
Measurements and statistical analysis
Demographic characteristics were as follows: Gender; age; family's religion; home location (urban/rural); parents' educational level; and family structure (whether students lived with both parents, a single mother/father, grandparents, or other relatives). Television watching and computer use: two variables were measured, (1) mean hours of TV watching or computer use from Monday through Friday, (2) mean hours of TV watching/computer use during weekends and holidays.
For time of usage of TV/computer on weekend and weekdays, we defined computer usage time as "Excluding the school's requirements (e.g. get information for doing homework), I usually used computer for playing games or chatting with someone", while watching TV was defined as "the time during which the TV was turned on and off on weekdays".
Adequate sleep (AS) was defined as 6–8 hours of sleep per night on more than four weekdays per week (this definition was drawn from the literature and real situations). Inadequate sleep (IAS) was defined as 6–8 hours per night on fewer than three weekdays. To obtain valid reporting of sleeping time per night and to ensure the reliability and validity of the measurement, the method we used was established in three stages. First, three junior-high and two high school adolescents were invited to participate in a pilot study that was a test of content validity. None of the students appeared to have any doubt about their own measurement of their sleeping times. Next, all participants were given instructions on counting their sleeping hours each night during the study period: for example, "if you go to bed at 11 PM and awaken at 6 AM, then you obtain total 7 hours of sleeping time". Finally, the students responded to a self-administered questionnaire with a Likert scale: "generally speaking, I sleep at least 6–8 hours each night", self-rating the frequency as one choice among "rarely, sometimes, usually, always". These frequencies were defined below the question: for example, "rarely" represented "I have 6–8 hours per weekday night on fewer than 2 nights"; "sometimes" represented "about three nights"; "usually" represented "4 nights"; and "always" represented "5 nights".
Health status was determined by two factors: (1) Body Mass Index (BMI), and (2) frequency of health insurance clams. Height and weight were measured when participants were dressed in light indoor clothing and without footwear. Height was measured to the nearest 0.1 cm, and weight to the nearest 0.1 kg. Each student's BMI was then calculated using the standard formula (weight (in kg) divided by height (in m2)). BMI was plotted on the age and sex-specific cutoff points to define the different body sizes of adolescents according to nationally accepted guidelines . Each student was classified as overweight (> 85th percentile for age and sex) or not overweight (> 5th percentile and < 85th percentile). For example, in Taiwan, a 15-year-old boy with a BMI > 23.1 was defined as overweight, whereas a 15-year-old girl with a BMI > 22.7 was considered overweight. Participants also reported how many times had they visited a doctor or been to a hospital for health problems during the past year, and explained the reason for their doctor visit.
Health-related behaviors were measured using the Adolescent Health Promotion (AHP) scale , which is considered valid and reliable. The AHP comprises 40 items assessing six dimensions of behavior: (1) nutritional (eating breakfast daily, eating 3 meals a day, drinking at least 1,500 cc of water daily, choosing foods with little oil, etc.); (2) social support (speaking to and sharing feelings with others, talking about personal problems with others, keeping in touch with relatives, etc.); (3) life-appreciation (making an effort to feel happy and content, degree of positive thinking, recognition of personal strengths and weaknesses and their acceptance, etc.); (4) health responsibility (reading food labels when shopping, washing hands before meals, standing or sitting up straight, etc.); (5) stress-management (smiling or laughing every day, making schedules, setting priorities, etc.); and (6) exercise behavior (exercising rigorously for 30 minutes at least 3 times per week, performing stretching exercises daily, etc.). The measuring instrument used to obtain the frequency of reported behaviors was a self-reporting Likert scale with a five-point response format: "never, rarely, sometimes, usually, always", with the rating score ranging from 1 to 5. The validity and reliability of the original AHP were assessed with a sample of 1128 Taiwanese adolescents and deemed to be satisfactory. Factor analysis yielded a six-factor instrument that explained 51.14% of the variance in the 40 items. Cronbach's alpha reliability coefficients were 0.93 for the total scale, and the alpha coefficients for the subscales ranged from 0.75 to 0.88 .
SPSS (Version 11.0) was used for all the data analyses. All tests were 2-sided and p-values less than 0.05 were considered statistically significant. Categorical data analyses (e.g. chi-square test, odds ratios and 95% confidence intervals) and multivariate analyses were applied to determine the relationships among AS, health status, health-related behaviors and their associated factors. Before any categorical data analysis, the 6 dimensions of the health-promoting behavior scale were classified as low frequency (if the value was below the mean score of each sub-scale) or high frequency (if the value was above the mean score of each sub-scale).