Autism/ASD
diagnosis rates in Australia

Bob Buckley
Computational Genomics Laboratory,
John Curtin School of Medical Research,
The Australian National University
Canberra ACT 0200 Australia

and

Autism Aspergers Advocacy Australia (A4)
No physical address
www.a4.org.au

convenor@a4.org.au




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Abstract
Reports show the number of people with an autism spectrum disorder
(ASD) diagnosis in various parts of Australia. The reports provide
prevalence and incidence data. The different types of data cannot be
compared directly.
The detailed age distribution of diagnoses provides the means to calculate
a ratio of the number of ASD diagnoses relative to the number of births
for a region. This metric estimates the number of people in the population
who have ASD. The current diagnosis rates in all regions of the country
suggest that around 1% of Australians will be diagnosed with an ASD.
ASD is life-long and can be severely disabling so this level of ASD
imposes a substantial burden on the community.
The age distribution of diagnoses can be further used to estimate for
planning purposes the number of people with an ASD diagnosis
(including Asperger’s syndrome) at important transitions, such as the how
many students diagnosed with ASD enter primary or secondary school.
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Introduction
Recently, the Minister for Health in Australia stated:
“There is no national data on the diagnostic profiles of people with autistic disorder and/or
Asperger’s syndrome and/or pervasive developmental disorders.” (Campbell 2003, page
16680)
and
“the Government does not plan to establish central registers for autism, Asperger's syndrome,
or pervasive developmental disorders. The provision of health and disability services for
people with these disorders lies with State and Territory jurisdictions as is the responsibility
for ensuring sufficient attention is made available from treating clinicians.” (Campbell 2004,
page 19851)
In the absence of official data, this paper collects the data available in Australia and
considers how it can be interpreted to help in planning services for people with an
autism spectrum disorder (ASD). In view of the varying classification criteria used by
different agencies, the term ASD will be used to cover autism, Asperger’s syndrome
and pervasive developmental disorders.
Prevalence data
In recent times, a number of surveys have collected data that includes people who
report having an r ASD diagnosis at a particular time.
Data of this type is called prevalence data. The prevalence of ASD is the number of
people in a population who have ASD at a specified time. In practice, we count the
number of people who report having an ASD diagnosis at a specified time. Typically,
the prevalence of autism/ASD is reported as the rate per 10 000 population; that is,
the number of people with an ASD among every 10 000 people if the people with
ASD are distributed evenly throughout the population.
Recent prevalence reports from parts of Victoria are summarised in Table 1 below,
which reveals a range of estimates from 27 per 10 000 to 54 per 10 000. Such a
variation in reported prevalence can make it difficult to interpret this type of
information.
“The prevalence of children with ASD in the region is at least 38 per 10 000 children aged 2–
5 years (inclusive). This is higher than the state-wide estimate of 27 per 10 000 children aged
0–6 in the Autism in Victoria Report (AIV Report)*.” (Burke, 2003).
Ms Burke observes:
“*The AIV Report calculation is not necessarily incorrect for the Eastern region when applied
to the 0-6 population. However, when the AIV Report’s calculation of 27 per 10 000 is
applied to the 2–5 age group in the region, the number estimated would be less than the actual
number of children with ASD aged 2–5.” (Burke, 2003)
The Chief Executive Officer of Gateway Support Services Geelong has remarked
“Gateways have identified 69 children between the ages of 2–6 years out of a total
regional population of 12,800 (2–6 years) children in 2000.” so ASD “may be as high
as 1:200” (The Melbourne Age, 27 October 2002; “A riddle wrapped in an enigma”,
The Melbourne Age, 16 November 2002) The author subsequently confirmed this
data with the CEO. The 2003 ratio of 1:200 is equivalent to 50 per 10 000, and this is
approximately the same as 54 per 10 000 reported in 2000 (see Table 1) so it appears
that the prevalence of autism in this region is relatively stable, at least according to
Gateway.
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In addition to being reported as rate per 10 000, prevalence can also be expressed as
numbers. For example Table 2 reveals in 1998 there were 10 500 children identified
with “autism and related disorders” in a total population of 2 627 500 children
between the ages of 5-14 years. Prevalence data of this type was obtained by the
author from The Australian Bureau of Statistics (ABS), derived from its Survey of
Disability, Ageing and Carers (SDAC). The SDAC is conducted every five years,
most recently in 1998 and 2003. Selected survey information, which is not part of the
usual ABS publications, is shown in Table 2 and Table 3 below respectively. These
data show the number of Australians who report having an ASD diagnosis to the ABS
has more than doubled in the 5 years from 1998 to 2003. Numbers rose from 13,200
to 30,400 over the period. In addition, two of every three people reported as having an
ASD diagnosis in 2003 were under 15 years of age, a point to be elaborated later.
A final source of prevalence data is the Department of Family and Community
Services (FaCS), whose minister reported (Patterson, pages 25845–6) that
(1) At 2 July 2004, there were 11 694 children under 16 years of age with their first medical condition
identified as autism whose carer received Carer Allowance. …
(2) At 2 July 2004, there were 2522 children under 16 years of age with their first medical condition
identified as Asperger’s syndrome/disorder whose carer received Carer Allowance. …
(3) Information on the number of families receiving Carer Allowance because they have a child with
Pervasive Development Disorder – Not Otherwise Specified (PDD-NOS) identified as F84.9 is not
available.
(4) Information on the number of families receiving Carer Allowance because they have a child with
another autism spectrum disorder identified as F84.1–F84.8 and not F84.5, is not available.
This shows there are at least 14,216 parents of children aged 0–15 years who report
that their child has autism or Asperger’s disorder to qualify for Carer’s Allowance. As
there are around 4.18 million children aged 0–15 years in Australia, prevalence would
be calculated as 34 per 10 000 for this cohort.
Incidence data
Data describing the number of new ASD diagnoses is called incidence data.
Recent publications (Baker, 2002; Glasson et. al., 2002; Glasson et. al., 2003; and
Williams et. al., 2002) report the number of people diagnosed with autism/ASD in
three different states / territories over a 12 month period. The South Australian
Autism Association provided its data for the years 2000 to 2002. Information from all
these sources is summarised in Table 4 below.
FaCS also supply incidence data. The minister reported (Patterson, page 25846) that
(5) For the period 2000 to 2003 there were 5125 claims for Carer Allowance in respect of a child
whose medical condition was listed as autism. …
(6) (a) For the period 2000 to 2003 there were 1944 claims for Carer Allowance in respect of a child
whose medical condition was listed as Asperger’s syndrome/disorder. …
(b) For the period 2000 to 2003 there were 1841 claims granted for Carer Allowance in respect of a
child whose medical condition was listed as Asperger’s syndrome/disorder. …
To summarise the FaCS information, there were at least 6966 Carer’s Allowances
granted in the four years 2000 to 2003, with a possible 7069 new diagnoses resulting
in applications for Carer’s Allowance over the period. The number of diagnoses may
well be higher, because uptake of Carer’s Allowance is not 100%, and this incidence
report is just for the two main diagnoses in the autism spectrum.
A source of confusion for many people is that the different reports of the prevalence
and incidence of ASD appear to show varying levels of autism in the community.
This report provides illustrative data to demonstrate why the results cannot be
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compared directly as they are currently presented, and it suggests a method for
comparing prevalence data with incidence data and with other prevalence data. As
discussed below, an advantage of this approach is that the available data can then be
used to estimate the number of people who are likely to need services in any age
group or at the various life-stage transitions.
Method
The effects of autism are said to last a lifetime and are expected (or required as part of
the diagnostic criteria for Autistic Disorder) to have been present from an early age. If
so, a person needs only one accurate diagnosis of ASD in their lifetime.
Because ASD is a lifelong condition, incidence and prevalence data can be compared
using some simple models of its occurrence in the population.
While much of the data above shows rising diagnosis rates, the Gateways and WA
data may be showing early signs that diagnosis rates may be levelling off. The
assumption that diagnosis rates are about to plateau simplifies modelling ASD
prevalence. The results may be conservative but they should still indicate the
minimum prevalence of ASD in the community.
Assuming that most people with ASD will be diagnosed at some time and the pattern
of diagnosis in recent times is close to stable, then the number of people in the
population with ASD is the number of diagnoses divided by the size of the population
from which they are drawn.
The results section below shows the estimates of ASD prevalence in the population
based on the incidence figures reported above. In practice, the prevalence would only
be measured in a population once all the diagnoses were made. The data suggests
these levels would only be observed once the children diagnosed recently reach an
age around twenty years.
Incidence data published by the Western Australian Autism Registry illustrates this
point. These data, shown in Table 5 and Figure 1, provide an age breakdown for those
diagnosed, and it is evident that some individuals are not detected until they reach
their teenage years. These people are not included in prevalence studies based on
young children. Thus, measured prevalence in young children will show the number
of children in an age range who have a diagnosis, but it will not show the prevalence
of ASD in the entire population.
The relationship between incidence and prevalence in young children up to six years
is illustrated graphically in Figure 2. For this cohort, about 35% of autism diagnoses
have yet to be made. A correction to this and other underreporting is presented below.
Results
From the 1998 and 2003 SDAC data (see Table 2 below national prevalence rates
were derived by dividing the prevalence numbers (total numbers of people with
autism) by the numbers in the population from which they were drawn (both
expressed as ‘000s). The results are shown in Table 6 below. The statewide
prevalence rates were derived in a similar fashion from Table 4, and are presented in
Table 8.
Table 8 reveals some discrepancies in the prevalence data reported by different
organisations. There are several reasons for this. Sometimes, personal preferences and
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diagnostic conditions skew the results. For example the Carer Allowance data from
FaCS identifies 14,216 children as having autism or Asperger’s syndrome out of 4.18
million children aged 0–15 years. This is fewer than 20,980 estimated from the ABS
from the SDAC 2003 figures in Table 3. (This estimate was obtained by adding the
numbers of identified children in the 0–4, 5–9, and 10–14 age ranges then taking one
fifth of those in the 15–19 age range.) The different estimates arise because some
families do not apply for Carer’s Allowance. Some applications relating to Asperger’s
syndrome are not granted. Only some of the conditions regarded as ASD are included
in this data.
The rate that Carer Allowance was granted from 2000 to 2003 indicates the
prevalence of autism and Asperger’s syndrome among children is around 67 per
10,000. The four-year incidence is divided evenly between the years and compared to
the annual birth rate.
A second reason for different prevalence reports can be found by analysing the age
distribution of diagnoses. The prevalence of people with a diagnosis of autism/ASD in
an age range does not accurately reflect the number of people who have autism/ASD.
The prevalence of ASD observed in young children significantly under-estimates the
actual prevalence. This point is illustrated in Table 5, where the “cumulative %”
columns allow us to determine the extent of the under-estimate in Western Australia
for 2001 and 2002. This column indicates the weight that should be given to the
prevalence figure, as it shows prevalence observed in each year of the 20 years age-
range as a fraction of the expected total prevalence of autism/ASD in the population.
Furthermore, weights can be calculated for a range of ages, not only for each year.
This is achieved by taking an average of the prevalence weights observed in the age
range. For example, in the 0–5 years age range in Table 5, the average in 2002 is [(0 +
.95 +14.22+37.44+55.92) divided by 5], or 29%. Thus, weightings can be applied to
the various age ranges sampled by different organisations, and more meaningful
comparisons can be made between them.
Applying this reasoning to the current data, Table 9 the displays the weightings for
the age ranges sampled by the three Victorian surveys, and the ABS and FaCS
Australian surveys. These weightings are reproduced in the second last column of
Table 10. The “real” prevalence is obtained by dividing the observed prevalence by
the weighted prevalence. This is represented as prevalence per 10 000 in the final
column of Table 10. Implications of this correction to the data are discussed below.
Discussion
The corrected prevalence shown in Table 10 is higher than “traditional” estimates of
prevalence of autism or ASD. Before discussing this discrepancy, there are some
sources of variation – to both corrected and uncorrected prevalence figures – that
should be acknowledged. In the first place, some irregularities in the data can be
accountable to changed reporting requirements. People who were 10–14 years of age
in the ABS data in 1998 are in the 15–19 year age range in the 2003 data. The number
reporting a disability dropped slightly (from 29.7 to 28.6 per 10,000) in this group
from 1998 to 2003. This effect is observed among people with other types of
disabilities when the survey method changes from parent reporting to self-reporting.
A second cause of variance in the data, mentioned previously, is that some families do
not apply for Carer’s Allowance, or their applications are rejected. Finally, some of
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the conditions regarded as ASD may not be completely included in this data. For
example, supplementary Western Australian information indicates that only a small
percentage of people with Asperger’s syndrome were identified. This is not typical of
the other data collected in Australia. This would also mean that even the corrected
prevalence estimates in this paper are conservative.
The above caveat notwithstanding, the analysis in this paper shows there is good
reason to suggest that some of the data used to plan service provision in the past may
underestimate the prevalence of ASD. A crucial message from this analysis is that
observed prevalence of ASD among young children varies enormously with the age
range. Table 5 shows the observed prevalence in different age ranges varies
enormously. For example, the rate that children have a diagnosis in the 2–5 year age
range is much lower than the rate of children aged 5 or 6 years entering primary
school. Put simply, the Western Australian data reveal most diagnoses of autism occur
at an early age, and so the prevalence estimates calculated above are conservative for
regions where diagnosis happens at a higher age.
Some reports suggest that some sectors may be gaining a finer appreciation of
prevalence data in recent years. In its report on Education of students with a disability
(December 2002), the Senate Employment, Workplace Relations and Education
Reference Committee said:
“Over the whole of Australia between 0.6 per cent and 1 per cent of children
are affected by autism or a related disorder”, p 54.
The results reported in Table 10 support the Committee’s conclusion, particularly in
light of their conservative nature mentioned above.
The data currently available does not help in understanding whether more people now
have autism or whether there was chronic under-diagnosis in the past. Nor does the
data tell us whether outcomes are improving. What we do know is that planning
places for individuals with ASD using prevalence measures requires a clear
understanding of the data.
Service providers and planners in WA can expect around 35 children aged 2–5 years
(or 42 children aged 3–5 years) per 10,000 children in the age range will be diagnosed
with ASD. WA schools can expect at least 64 children with an ASD diagnosis per
10,000 primary aged students and 79 per 10,000 high school students. Nationally, the
ABS SDAC 2003 found 62.9 children aged 5–9 years and 80.2 children aged 10–14
years report having an ASD diagnosis (see Table 7).
The greatest increase shown in the nationwide data from the ABS is the increase in
the number of adults reported as having a diagnosis of ASD. From 1998 to 2003, the
number of people reported as having an ASD diagnosis rose from an estimated 1,700
(from Table 2 calculate 13,200 less 1,000+10,500) to 10,100 (from Table 3 calculate
3,900+6,200). This brings the rate of reported ASD diagnosis among Australian
adults closer to the rate expected in the population 15 years ago (Wing 2002).
Whether the diagnosis rate for ASD in adults will reach levels well over 10 per
10,000, significantly higher than levels previously expected, has yet to be seen.
Preliminary data from FaCS suggests that 3 of every 4 people with ASD now reaching
age 16 years, the eligible age, are granted a Disability Support Pension. The large
number of pensions granted shows existing services do not succeed in preparing
children to participate economically in their community.
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Conclusion
Counting young children with an ASD diagnosis does not measure the prevalence of
ASD in the population. The number of children reported as having received an ASD
diagnosis varies considerably with the age range being observed.
The diagnosis rates for autism and ASD have risen significantly in Australia, as they
have elsewhere in the world over the last 10 to 15 years.
Nevertheless, the analysis in this paper suggests that the ASD diagnosis rate
documented in recent years is reasonably consistent when the various observations are
normalised.
Acknowledgements
Ken Black from the ABS provided the data on autism and related disorders from the
ABS SDAC. He provided the data from the 2003 SDAC in a very timely fashion.
Andrew Brien and Judy Brewer Fischer strongly encouraged writing this paper.
References
H. Baker, (April 2002) A Comparison Study of Autism Spectrum Disorder Referrals
1997 and 1989, JADD, Vol. 32, No. 2.
A. Burke (Nov 2003) Autism in the Eastern Region: An Investigation into the
Experiences of Transitions for Children with Autism (0–6 years) and Their Families
during Early Childhood, intern report prepared for Dympna Beard M.P.
Senator Ian Campbell, Senate Hansard, 29 October 2003, see
http://www.aph.gov.au/hansard/senate/dailys/ds291003.pdf
Senator Ian Campbell, Senate Hansard, 10 February 2004, see
http://www.aph.gov.au/hansard/senate/dailys/ds100204.pdf
S. G. Crewther, M. J. Goodyear, E. L. Bavin, M. L. Lawson, S. A. Wingenfeld, D. P.
Crewther (2003) Autism in Victoria: An Investigation of Prevalence and Service
Delivery for Children Aged 0–6 Years, Department of Human Services, Melbourne,
Victoria (allegedly available from www.dhs.vic.gov.au). A summary of the report is
available at
http://www.ecis.vic.gov.au/bhcv2/bhcattach.nsf/images/ecis_autism_in_vic/$file/autis
m_in_vic.pdf.
Glasson E, Chauvel P, Cohen D, Cook H, McMullin B, Smith K, Tender J,
Wolfenden H, Wray J (2002). WA Register for Autism Spectrum Disorders – 2001
report. Perth, Western Australia, obtained from
http://www.autismwa.org.au/wa_autism_register.htm.
Glasson E, Campbell R, Chauvel P, Cohen D, Cook H, McMullin B, Tender J,
Wolfenden H, Wray, J. (2003). WA Register for Autism Spectrum Disorders – 2002
Report. Perth, Western Australia, obtained from
http://www.autismwa.org.au/wa_autism_register.htm.
Senator Kay Patterson, Senate Hansard, 5 August 2004, see
http://www.aph.gov.au/hansard/senate/dailys/ds050804.pdf
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The Melbourne Age (27 October 2002), “Autism finding: call for action”, see
http://www.theage.com.au/articles/2002/10/26/1035504923731.html ; also in “A
riddle wrapped in an enigma”, The Melbourne Age, (16 November 2002), see
http://www.theage.com.au/articles/2002/11/15/1037080914171.html
K. Williams, L. Bartak, C. Mellis (November 2002), The Incidence of Autism
Spectrum Disorders in NSW, World Autism Congress, Melbourne.
L. Wing, D. Potter (2002) The epidemiology of autistic spectrum disorders: is the
prevalence rising? Ment Retard Dev Disabil Res Rev; 8: 151-161.
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Source Age
range
ASD per
year
(years)
10,000
Gateway Support Services
2–6
54
2000
Autism in Victoria (Crewther, 2003)
0–6
27
2003
Eastern Region (Burke, 2003)
2–5
38
2003
Table 1 ASD prevalence reports (per ’000) in Victoria

ASD
Population
Age range
’000s
’000s
** this value has a
0–4 **1.0
1,284.8 high standard error.
5–14 10.5
2,627.5
Total 13.2
18,660.6
Table 2 ASD national prevalence (numbers) from ABS 1998 SDAC

age range
Male
Female Total ASD Population
(years)
’000s
’000s
’000s
’000s
0–4 1.2
0 1.2
1,243.5
5–9 6.5
1.8
8.2
1,309.9
10–14 8.2
2.6
10.8
1,347.3
15–19 3.3
0.7 3.9
1,363.1
20+ 6.1
0.1
6.2
14,547.3
Total 25.3
5.1
30.4
19,811.1
Table 3 ASD national prevalence (numbers) from ABS 2003 SDAC

State Year
Number
with ASD
ACT (Baker, 2002)
1989 9
1997 27
WA (Glasson et al., 2002)
1999 159
2000 173
2001 204
WA (Glasson et al., 2003)
2001 211
2002 211
NSW (Williams et al.; 2002)
2000
758
SA
2000 115
2001 206
2002 270
Table 4 Statewide incidence data.

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