A system dynamics modelling approach to studying the increasing prevalence of people with intellectual developmental disorders in New South Wales
Authors: Lee Lynette , Heffernan Mark , McDonnell Geoffrey , Short Stephanie D. , Naganathan Vasi 
Published: October 2015

Objective
The aims of this study were to estimate the prevalence count of people with intellectual developmental disorders (IDD) in New South Wales (NSW) in 2003, by age groups, and to forecast their prevalence until 2043.

Methods
Administrative data obtained from NSW government departments of education, pensions, health and disability were used to profile the number of people whose characteristics met the criteria for ‘intellectual developmental disorders’ who had received services in 2003. These figures were compared with published tables of NSW data from the national self-report Survey of Disability, Ageing and Carers (SDAC) of 2003 to estimate the likely prevalence of people with intellectual developmental disorders, by age groups in that year. The results were then used as baseline figures in a computational system dynamics model of the aging chain of people with these disorders, built to project prevalence to 2043.

Results
The number of people who met the criteria for having intellectual developmental disorder in NSW in 2003 was estimated to be 57000 (a ratio of 85 per 10000), with 32000 aged 0–15 years, 15000 aged 16–39 years, 9000 aged 40–64 years and 1000 aged 65+ years. Using these figures as baseline, the computer simulation predicted a total increase to 77225 people in 2013 and 135905 people by 2043. By 2043, the number of children with intellectual developmental disorders will have doubled, from 32000 to 59480, and the number of adults will have tripled, from 25000 to 76420.

Conclusions
This modelling technique forecast an increase in the prevalence count of people with intellectual developmental disorders in NSW over the period 2003–43 from 57000 (85 per 10000) to 135905 (135 per 10000). These predictions may have important implications for the planning of specialist health services for this group of people.

What is known about the topic?
The prevalence ratio of people with intellectual developmental disorders is quoted at lying between 1% and 2% of the Australian population, depending on the definition adopted. It is known that life expectancy for this group of people is increasing. Many people with intellectual developmental disorders have multiple service demands and there is a need to understand the prevalence count in various age groups in order to plan effectively for their health service needs.

What does this paper add?
This paper confirms a NSW prevalence ratio of people with intellectual developmental disorders of approximately 0.85% for the purposes of specialist health service planning at the beginning of the 21st century, and this is predicted to increase to 1.35% over a 40-year period. The paper demonstrates that there will be significant growth in the number of adults surviving to old age between 2003 and 2043.

What are the implications for practitioners?
It is known that as people with intellectual developmental disorders age, their health promoting care needs increase, as do their dependencies on special supports. Planning for the allocation of resources associated with the welfare and healthcare of people with intellectual developmental disorders may need to be focused on this anticipated increase in the number of older people with the condition.