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One purpose of a longitudinal study is to gain a better understanding of how an outcome of interest changes among a given population over time. In what follows, a trajectory will be taken to mean the series of measurements of the outcome variable for an individual. Group-based trajectory modelling methods seek to identify subgroups of trajectories within a population, such that trajectories that are grouped together are more similar to each other than to trajectories in distinct groups. Group-based trajectory models generally assume a certain structure in the covariances between measurements, for example conditional independence, homogeneous variance between groups or stationary variance over time. Violations of these assumptions could be expected to result in poor model performance. We used simulation to investigate the effect of covariance misspecification on misclassification of trajectories in commonly used models under a range of scenarios. To do this we defined a measure of performance relative to the ideal Bayesian correct classification rate. We found that the more complex models generally performed better over a range of scenarios. In particular, incorrectly specified covariance matrices could significantly bias the results but using models with a correct but more complicated than necessary covariance matrix incurred little cost. [ABSTRACT FROM AUTHOR]

Background: Gastroenteritis is an important cause of morbidity in both adults and children worldwide. Although the burden of morbidity and mortality is highest in developing countries, gastroenteritis is still a significant cause of morbidity in Australia, particularly in young children. The aims of the present study were to determine the incidence of gastroenteritis among 4–6-year-old children in South Australia, to describe the impact on daily activities of the child and caregiver and estimate the cost of gastroenteritis in this population. Methods: A diary study of drinking water consumption and gastrointestinal and respiratory symptoms was undertaken among 965 rural children in South Australia over a 6-week period during February–May 1999. Data on the response to gastrointestinal episodes were collected by telephone surveys midway and at the end of the 6-week period. Costing was based on the methods of Hellard et al. Results: The incidence of community gastroenteritis among 4–6-year-old children was 5.53 episodes per child-year (95% confidence interval: 4.87–5.80). The costs associated with childhood gastroenteritis were estimated to be between $AU16 858 360 and $AU22 477 814 per annum. Conclusions: The incidence of community gastroenteritis among the 4–6-year-olds in the South Australian population is high and it costs the community a significant amount of money in terms of direct medical costs and time lost by parents and carers looking after their ill children. [ABSTRACT FROM AUTHOR]

Study Objectives: To examine if social networks with children, relatives, friends, and confidants predict survival in older Australians over 10 years after controlling for a range of demographic, health, and lifestyle variables., Design: Prospective longitudinal cohort study (the Australian longitudinal study of aging), Setting: Adelaide, South Australia., Participants: 1477 persons aged 70 years or more living in the community and residential care facilities., Main Results: After controlling for a range of demographic, health, and lifestyle variables, greater networks with friends were protective against mortality in the 10 year follow up period. The hazard ratio for participants in the highest tertile of friends networks compared with participants in the lowest group was 0.78 (95%CI 0.65 to 0.92). A smaller effect of greater networks with confidants (hazard ratio = 0.84; 95%CI = 0.71 to 0.98) was seen. The effects of social networks with children and relatives were not significant with respect to survival over the following decade., Conclusions: Survival time may be enhanced by strong social networks. Among older Australians, these may be important in lengthening survival.