Fractional polynomials and model selection in generalized estimating equations analysis, with an application to a longitudinal epidemiologic study in Australia

In epidemiologic studies, researchers often need to establish a nonlinear exposure-response relation between a continuous risk factor and a health outcome. Furthermore, periodic interviews are often conducted to take repeated measurements from an individual. The authors proposed to use fractional polynomial models to jointly analyze the effects of 2 continuous risk factors on a health outcome. This method was applied to an analysis of the effects of age and cumulative fluoride exposure on forced vital capacity in a longitudinal study of lung function carried out among aluminum workers in Australia (1995-2003). Generalized estimating equations and the quasi-likelihood under the independence model criterion were used. The authors found that the second-degree fractional polynomial models for age and fluoride fitted the data best. The best model for age was robust across different models for fluoride, and the best model for fluoride was also robust. No evidence was found to suggest that the effects of smoking and cumulative fluoride exposure on change in forced vital capacity over time were significant. The trend 1 model, which included the unexposed persons in the analysis of trend in forced vital capacity over tertiles of fluoride exposure, did not fit the data well, and caution should be exercised when this method is used.

Communicating the ethos of codes of ethics in corporate Australia, 1995-2001: whose rights, whose responsibilities?

This paper considers the commitment to business ethics of the top 500 companies operating in the Australian private sector and communicates the results of a longitudinal study conducted from 1995 to 2001. Primary data was obtained (in 1995 and again in 2001) via a self-administered mail  questionnaire distributed to a census of these top 500 Australian  companies. This commitment of each company to their code of ethics was indicated and measured via a range of methods used by organizations to communicate the ethos of their codes to employees. Just as they were in  1995, it would appear that companies in 2001 still are good at ensuring that their rights are protected, but at the same time they do not seem to take on the responsibility to ensure that employees'' rights are just as well protected. This double standard leads to cynicism towards the current business ethics processes inherent in Australian companies.

Income differences in food consumption in the 1995 Australian national nutrition survey

Objective: To assess the relationships between an index of per capita income and the intake of a variety of individual foods as well as groups of food for men and women in different age groups. Design: Cross-sectional national survey of free-living men and women. Subjects: A sample of 5053 males and 5701 females aged 18 y and over who completed the Australian National Nutrition Survey 1995. Methods: Information about the frequency of consumption of 88 food items was obtained. On the basis of scores on the Food Frequency Questionnaire, regular and irregular consumers of single foods were identified. The relationships between regularity of consumption of individual foods and per capita income were analysed via contingency tables. Food variety scores were derived by assigning individual foods to conventional food group taxonomies, and then summing up the dichotomised intake scores for individual foods within each food group. Two-way ANOVA (income age group) were performed on the food variety scores for males and females, respectively. Results: Per capita income was extensively related to the reported consumption of individual foods and to total and food group variety indices. Generally, both men and women in low income households had less varied diets than those in higher-income households. However, several traditional foods were consumed less often by young high-income respondents, especially young women. Conclusions: Major income differentials in food variety occur in Australia but they are moderated by age and gender. Younger high-income women, in particular, appear to have rejected a number of traditional foods, possibly on the basis of health beliefs. The findings also suggest that data aggregation has marked effects on income and food consumption relationships.

The Age (Education section) 2003 -

Dr Marcia Devlin is an educational psychologist and Professor of Higher Education Research at Deakin University. Marcia has a broad and extensive publication record in the scholarship of teaching and learning that incorporates academic development, student learning support and the use of information and communication technologies in teaching and learning. She writes regularly for The Age and Campus Review newspapers.