Study Population
The HWR is a prospective cohort study of community-dwelling older adults. It commenced in 2006 as a postal survey of a representative national sample aged 55-70, randomly selected from the New Zealand electoral roll. Of the original cohort (N=6662), 2632 consented to be invited to participate in subsequent waves. Of these, 1609 (41%) were lost to follow up over the five waves of the study (212 to death, and remaining unknown). Data has been collected biennially. The core questionnaire assesses domains of health and well-being; family and social support; work and retirement; financial well-being; and cultural identity.
Measures
Multimorbidity
Participants with MM were selected based on their response to a question ‘Has a doctor, nurse or other healthcare worker told you that you have any of the following health problems (a list of conditions was provided)? We selected nine groups of diseases for this analysis according to the availability of data in all waves including: heart disease, stroke, other neurologic diseases (epilepsy, Parkinson, migraine headache, Alzheimer/dementia), musculoskeletal (arthritis, osteoporosis, hip/knee replacement), diabetes mellitus, respiratory diseases (Chronic Obstructive Pulmonary Disease (COPD), asthma), chronic liver conditions (cirrhosis), cancer, and mental disorder (depression, anxiety and other mental diseases). Participants were classified as 1) “MM participants”; those who answered “yes” to at least two diseases in this list, 2) “Non-MM participants” defined as participants who were without MM in any of the study waves and the baseline. MM was also categorised as no. of chronic diseases (up to 3+). We also analysed the HRQOL over the time according to the number of chronic diseases (0, 1, 2, 3+).
Health-Related Quality of Life
HRQOL was assessed using the SF12, a short version of the HRQOL measures developed within the framework of the Medical Outcome Study (11). SF-12 items are rated 1-5 and standardised norm based orthogonal factor weights calculated to form a Physical Component Score [Physical Component Scale (PCS); positive weights for physical functioning (2 items), role physical (2 items), pain and general health] and Mental Component Score [Mental Component Scale (MCS); positive weights for vitality, social functioning, relationships (2 items) and mental health (2 items)] (12). Higher values represent a higher QOL.
Demographic Variables
Age, provided as a continuous variable, was categorised as 55-64 years and 65 and over. Marital status was considered as two groups: married/living with a partner, divorced/ separated/single/ widow; ethnicity was classified as priority ethnic groups in New Zealand, Maori and non-Maori (Europeans, Asians, Pacific people and other ethnicities). Socioeconomic status indicators included educational qualification categorised as no secondary, secondary, post-secondary and tertiary; and annual personal income (0- 25000, 25001-50000, 50001-70000, >70000 NZ$). Smoking was defined by asking respondents to identify themselves as a regular smoker or not.
Behavioural Variables
Alcohol consumption was assessed by the frequency of drinking, from never, through up to four times a month, to two or more times a week, and then classified into two groups; regular consumption (2 or more drink per week) and occasional consumption (less than two drinks) per week. Physical activity was measured by the number of moderate activities including brisk walking, and vigorous activity in the last seven days that was categorised into two levels: two or more times per week, once per week/none. Body Mass Index (BMI, kg/m2) was measured only in 2008 and categorised as normal weight (<25), overweight (25-29.9), obese (≥30).
Clinical variables
Hypertension and eye problems (including cataracts, glaucoma and blindness) were included as dichotomous variables (yes/no), based on the responses to a question about the doctor-diagnosed health problems. Vision problems were also measured by asking about the ability to see an ordinary newspaper (with glasses or contact lenses if worn).
Statistical analysis
Differences in the characteristics of people with and without MM at baseline were determined by Student’s t-test for continuous variables and the chi-square test for categorical variables. Generalized Estimating Equations (GEE) with an exchangeable correlation matrix and robust standard errors were used to analyse the data. The results are presented as estimates (with 95 percent confidence intervals) of the difference between the two groups (those with/without MM). The difference of two components of the HRQOL; PCS, MCS, between groups was estimated after adjusting for baseline values in the first model and baseline values for age, sex, ethnicity, education, marital status, hypertension, alcohol consumption, smoking, and physical activity in the second model. We repeated the analysis according to the number of chronic conditions separately.
Data were analysed using the STATA statistical package Version14, all estimates were reported with 95% confidence interval and a significance level 0.05.