February 2012
Western Australia Tomorrow
Population Report No. 7, 2006 to 2026
Forecast Profile
Bridgetown-Greenbushes (S)
Local Government Area
Population Report No. 7
Western Australia Tomorrow
Forecast Profile for the
Bridgetown-Greenbushes (S) Local
Government Area
Published by the
Western Australian Planning Commission
140 William Street
Perth, Western Australia 6000
Authors: Tom Mulholland and Anna Piscicelli
Disclaimer: Any representation, statement opinion or advice expressed or implied in this publication is made in good faith and on
the basis that the government, its employees and agents are not
liable for any damage or loss whatsoever which may occur as a result of action taken or not taken, as the case may be, in respect of
any representation. statement opinion or advice, referred to herein.
Professional advice should be obtained before applying the information contained in this document to particular circumstances.
Foreword
Western Australia Tomorrow is a set of forecasts1 based on trends
since the 1980s. The forecasts represent the best estimate of future
population size if trends in fertility, mortality and migration continue. They use the latest information about changes in trends. In
some cases these have occurred since the 2006 base year.
Trend forecasts are used in a number of ways. One of them is to
identify those futures which we wish to build upon and some that
we would rather avoid. As a result government has adopted plans
and strategies that are expected to change future trends. These include Directions 2031 and Beyond, Pilbara Cities and Supertowns.
Each of the plans and strategies has included a forecast of future
population.
The forecasts within these plans and strategies differ from WA Tomorrow in a number of ways. In some cases, such as Directions
2031, the aggregate forecast has been consistent with WA Tomorrow. The emphasis in this plan is on meeting the requirement to
find room for future population growth while maintaining local environments and valued quality of life. In other cases the forecasts
represent an aspirational target which is seen as beneficial for the
communities involved. The emphasis may not be on the forecast
but rather on what changes may be needed to change future population. As a result the forecast is about direction and not the
ultimate size of population.
Future WA Tomorrow forecasts will incorporate the changes achieved
through these plans and strategies. Sometimes it will be easy to
know how to incorporate the different views of the future. Readers
will need to fully understand what a particular plan or strategy is
trying to achieve and make an assessment on the relevance of the
plan or strategy.
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
Overview
have been broken into five bands, each with
2 000 simulations. We have published the meThis population forecast is one of a set of dian value of each band to give 5 forecasts.
forecasts for each Local Government Area in
Western Australia.
Band A contains the lowest simulations.
Band E has the highest simulations. The
These forecasts have been prepared using forecast for Band C is also the median value
10 0002 slightly different simulations. The for all forecasts as it is the middle band. The
simulations emulate the variability that is Band C forecast is comparable with the preshown in past data. The simulations have vious WA Tomorrow (2005) publication.
been sorted by the size of population. They
Figure 1: Forecast of total population
When assessing the probability of a forecast for a single region, users typically take
each forecast to be independent.3 Past forecasts have shown that there will be individual
shires where the top of the range is easily met.
The hard part is working out if BridgetownGreenbushes will be a region that does not
follow the trend.
aware of such initiatives and the impact that
they may have in the future. In some cases
it may help to use any population scenarios
that are included with such projects.
Population Change
Figure 1 shows each of the bands within the
forecast. The bands have been coloured4 and
In addition to past instability, all levels of the median value of each band as at 2026 has
government have the task of changing trends been printed on the chart.
through planning processes. Users should be
WA Tomorrow Population Report No. 7
1
Forecast Profile
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
−0.1% and a high of 2.6%. The number
of births is significantly higher than deaths.
Births are also a more important component
than net migration.
The range of these forecasts suggest that
users need to be careful when making decisions based on these forecasts. There is
a signifcant variation between Band A and
Band E that should be taken into account.
Table 2 shows the range of AAGRs for 20,
15 and 10 years. To put these figures in
context the rates have been compared with
the Australian AAGRs prior to the recent
student induced5 record growth rates. The
Bridgetown-Greenbushes AAGRs are higher
than the Australian experience. By WA standards they can be considered to be good
growth.
One way of looking at growth is to calculate
the average annual growth rate or AAGR.
The AAGR is the constant rate of change
that is required to reach the size of population in a particular year. It is likely that in
the long term there will be population growth
in Bridgetown-Greenbushes. The average annual growth rate (AAGR) for Band C is 1.5%.
This compares with a lowest change rate of
Figure 2: Demographic Accounts
Demographic Accounts
A waterfall chart (Figure 2) gives a visualisation of the cumulative effect of each component. Within each band the components
have been ordered by the absolute size of their
impact. The largest impacts are shown last.
The cumulative effect of all components is
equal to population change over the 20 year
period. The cumulative values6 are printed in
These forecasts have been prepared using a
cohort component model that includes information about migration flows in and out of
regions within Australia, net migration into
Australia, births and deaths.
Forecast Profile
2
WA Tomorrow Population Report No. 7
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
red alongside the last component. The bands
are ordered so that the lowest band is on the
left and the highest is on the right. A dashed
line indicates no population change. This allows the user to see the overall balance of the
components.
very large. Instead of a reduction in fertility, this forecast continues the current high
rates of fertility for the horizon of the forecast. This means that births are playing a
more significant role in population change,
than they have previously.
A feature of these forecasts is an increase in
the assumptions for fertility and overseas migration into Australia. The change in fertility
rates between those used here and the ones
used in the 2005 edition of WA Tomorrow is
The main component changed from intrastate
migration for Band A, to overseas migration
for Band E. As expected the birth and death
components were the two most stable aspects
of the model.
Figure 3: Boxplots of demographic components
An alternative way at looking at the components is by the use of boxplots. These visualisations allow the user to see the distribution
of values in each band.
The boxplots show the structured way in
which the demographic components change
each other. There are distinct differences between each of the bands. Close examination
of a single band shows that the range of valThe dark-line in the centre of the boxplot is ues used can be quite large.
the average (median) value of that band. 50%
of the values are within the box. The whiskers
Age and Sex Structure
attached to the box have a range that is 1.5
times that of the mean to box edge. Finally Changes have been made in this forecast to
outliers are shown as solid dots.
WA Tomorrow Population Report No. 7
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Forecast Profile
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
improve the accuracy of the age and sex
structure. A detailed analysis of past forecasts suggested that the difference between
the forecast and what occurred was substantial. Figure 4 gives a visualisation of the dynamics of the age structure throughout the
forecast. The figure is split with both the
top and bottom parts sharing the same x axis
(5 year age groups.)
2026. Each polygon has been hatched and
coloured. Cross hatching indicates the overlap between census years. The bottom part
shows the ranges (all bands) of the AAGR for
each age group. If the bottom chart is relatively flat it indicates that all age groups are
changing at the same rate. In this case the
ranges will all share the same shape. That is
peaks and troughs will all remain in the same
age. If AAGRs have a high rate of change
The top part overlays the ranges (all bands) then the top chart will spread out revealing
for each age group as a polygon. There is the population increase.
a polygon for each census year from 2006 to
Figure 4: Age structure for 2006, 2011, 2016, 2021 and 2026
lot of overplotting on these charts.
If the AAGRs are not flat they indicate that
some age groups are changing faster than others. This is nearly always the case for the
older age groups. This part of the chart enables the user to gauge how individual age
groups are changing in comparison to each
other.
The AAGRs for young people (aged 0 to 19)
centres around 1.1%. Those of working age
(20 to 64) have a rate of about 0.9%. Older
people have a rate closer to 5.1%.
Analysis of the output from this model shows
During the period of these forecasts the age the forecasts do not exhibit the universal age
7
structure remains familiar. There is quite a creep that was a prominent feature of previ-
Forecast Profile
4
WA Tomorrow Population Report No. 7
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
ous forecasts. This was most noticeable in areas where there were strong migration trends
that suggested a stable age and sex structure. These were typically associated with
mining employment or attendance at an educational institution. In past forecasts an adhoc adjustment was made using past trends
of changes in the structures. In line with
academic work8 the introduction of migration
flows appears to have resulted in improved estimates of age and sex.
Details of the assumptions used at a State
level are included in the summary publications.
Mortality assumptions in BridgetownGreenbushes (S) are derived for both the
Indigenous population as well as the nonIndigenous population. While there is no
way to accurately determine the local mortality rates for Indigenous people, it is well
known that there are very significant differences between the two groups. The forecasts
do this because in areas with substantial Indigenous populations, even a crude estimate
of Indigenous mortality can significantly improve the forecast.
Assumptions used in the model for
Bridgetown-Greenbushes (S)
The assumptions for each area have been created using both local and State data. It has
been shown that local forecasts of population
are improved by adjusting each sub-region so
that the sum of the components results in the
same outcome as the State estimate.
Figure 5 is for the total population and takes
account of all of the adjustments made in the
forecast process. It is usual to transform the
rate by applying a log function. This enables
the reader to see the subtle changes that are
happening.
Figure 5: Age specific mortality rates for the years 2011 to 2012, 2015 to 2016 and 2025 to
2026
WA Tomorrow Population Report No. 7
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Forecast Profile
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
Figure 6: Age specific fertility rates for the years 2011 to 2012, 2015 to 2016 and 2025 to
2026
The use of a State-wide assumption was made
after looking at the spatial variations in mortality. A recent Australian Bureau of Statistics publication9 produced an article looking
at remoteness areas and found that there was
a difference between remote and non remote
areas. There seems to be a similarity between
this finding and the one used in this publication. That is remoteness is also associated
with higher proportions of Indigenous people.
Local fertility assumptions were made by
identifying regions that had a similar fertility pattern. For example the outer areas of
Perth have higher levels and a younger peak
of age specific rates than the inner areas of
Perth. Likewise urban centres in the country had lower rates and an older peak in age
specific rates than other country areas.
The rates for these groupings were used for
all areas within the grouping. The variation
This topic is part of ongoing discussion with in rates between individual areas is incorpothe Australian Bureau of Statistics to im- rated in the uncertainty shown in Figure 6.
prove the quality of Indigenous statistics. It
does not appear that there is an obvious way A single assumption was used for the Indigeto spatially vary mortality rates at the mo- nous mothers. The net effect of both assumptions have been combined in Figure 6.
ment.
Forecast Profile
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WA Tomorrow Population Report No. 7
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
Figure 7: Age specific migration rates for the years 2006 to 2026
Rates for the three migration types used in
the model are shown in Figure 7. These
are net migration rates. They are helpful
in understanding the change in population
size. However, they also hide most of the migration that actually occurs. Approximately
10% of the total population will have moved
to Bridgetown-Greenbushes from elsewhere
in WA, each year. Interstate migrants add
another 1%. The figure for net overseas migrants is not calculated using flows. Therefore the inward component is unavailable.
However as with the other migration flows it
is probably much larger than the net migration estimate.
forecasts.
The estimates for overseas migration may
incorrectly contain movements within Australia. This is because there is no direct
way of estimating who has moved overseas
from Bridgetown-Greenbushes. It could be
that people who have left, failed to identify Bridgetown-Greenbushes as their previous address on the Census form. This will
have most impact for groups, such as young
males, who tend to either be missed or fail to
answer questions on the Census form.
Estimates of overseas migration have been
made using linear regression. From this confidence intervals have been used to estimate
the levels of uncertainty. However past levels
of overseas migration have been influenced by
changes to government policy. These changes
have often been sudden and dramatic. This
type of uncertainty is not included in these
WA Tomorrow Population Report No. 7
7
Forecast Profile
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
Notes
to expect them to all be average. For example if you throw six dice, you expect that one
of the dice will roll a six quite quickly. Indeed
there is a 66% chance that it will be thrown
1
As with the previous set of projections in the first 6 rolls and a 90% probability that
the terms forecast and projection have the a five or six will be thrown.
same meaning within this document.
4
These colours have been selected so that
2
While this may seem like a large num- people with some of the more common types
ber of runs, it is in fact less than would be of colour blindness can distinguish the differrequired to be able to say with any degree ences.
of certainty that an alternative run of 10 000
5
Recent population growth in Australia
would not give significantly different figures.
has been connected with changes to the way
For example, if we tried to select a combi- in which the population is counted. The
nation of factors that ensured each year had change mainly relates to people who are in
five runs that covered the possibilities be- Australia for longer than 1 year, but do
tween low and high we would need 95 tril- not have permanent residence status, such as
lion unique runs to cover all possible permu- overseas students. For a while a boom in vocational education encouraged high levels of
tations.
students hoping to gain permanent residence
In the first year there would be five pos- in Australia. Changes to migration processes
sibilities. In the second year each of the five in 2010 appear to have reduced the numbers
options have 5 more options. This is nor- of students.
mally shown as 52 or 5 to the power of 2 which
6
Since the forecasts are sorted in order to
equals 25. The next year we have 53 or 5 to
the power of 3 which equals 125. To do the rank the runs for each year, the median val20 years in the forecast we need 521 or 95 367 ues of the demographic components are not
required to add to the size of the total popula431 640 625 or 95 trillion.
tion. For smaller areas the differences may be
Running this many simulations is not large enough to notice. However the overall
possible. Our 10 000 simulations represent a pattern will be correct. Using an individual
sample which we can use as representative of run could produce a run that was not repthe minimum of 95 trillion possibilities. Us- resentative of the change from band to band,
ing a sample size calculator the best we can although the sum of the components could be
expect of the mean of all simulations is that guaranteed to total correctly.
they are within 1% plus or minus and we are
7
Age creep is the way the existing age
about 95% confident about that. The use of
1 000 runs changes that to 3%. The reality structure appears to age in place. That is afis that both figures are much larger as there ter 5 years a peak that was at age 20 now
is no way that we only need 476 trillion sim- peaks at age 25, suggesting that the populaulations. Most of them will be duplicates. It tion is stable and therefore the 20 year olds
seems likely that we are probably within the are most likely the same 15 year olds.
ball park and not much else.
8
Isserman, A. M. (1993). The Right
3
This forecast is part of a series. All of People, the Right Rates Making Population
them are related to each other. Some will be Estimates and Forecasts with an Interregional
higher and some will be lower. It is unrealistic Cohort-Component Model. Journal of the
Forecast Profile
8
WA Tomorrow Population Report No. 7
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
American Planning Association 59(1): 45-64. Scale Population Projection Models by Aggregation and decomposition. Laxenburg,
Kupiszewski, M. and P. Rees IIASA. p. 60.
(1999). Lessons for the projection of internal migration from studies in ten European
Wilson, T. and M. Bell (2004). Comcountries. Statistical Journal of the United parative empirical evaluations of internal miNations 16 281 - 295.
gration models in subnational population projections. Journal of Population Research
Rees, P. (1985). Developments in the 21(2): 127-160.
modelling of spatial populations. Population
9
Structures and Models: Development in spaABS (2011).
Deaths, Austial demography. R. Woods and P. Rees. tralia.
2010, 3302.0.
Canberra, AusLondon, Allen & Unwin: 97-124.
tralian Bureau of Statistics.
Website:
www.abs.gov.au/ausstats/[email protected]/mf/3302.0
Rogers, A. (1975). Shrinking Large-
WA Tomorrow Population Report No. 7
9
Forecast Profile
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
Table 1: Population Forecasts by Bands 2006 to 2026
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
A
4 200
4 100
4 000
3 800
3 700
3 700
3 600
3 600
3 600
3 700
3 700
3 800
3 800
3 900
3 900
3 900
4 000
4 000
4 100
4 100
4 100
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
5
5
5
B
200
200
200
200
200
300
300
400
400
500
500
600
600
700
800
800
900
900
000
000
100
C
4 200
4 300
4 300
4 500
4 500
4 600
4 700
4 800
4 900
5 000
5 000
5 100
5 200
5 200
5 300
5 300
5 400
5 500
5 500
5 600
5 600
D
4 200
4 300
4 500
4 700
4 800
5 000
5 100
5 200
5 300
5 400
5 500
5 500
5 600
5 700
5 800
5 800
5 900
6 000
6 100
6 100
6 200
4
4
4
4
5
5
5
5
5
6
6
6
6
6
6
6
6
6
6
6
6
E
200
400
700
900
200
400
600
800
900
000
100
200
200
300
400
500
600
700
800
800
900
Table 2: AAGRs and Australian Ratio by Bands, 2026, 2021 and 2016
AAGR
A
B
C
2026 −0.1 1.0 1.5
2021 −0.4 1.0 1.7
2016 −1.1 0.8 1.9
Forecast Profile
D
E
2.0 2.6
2.3 3.0
2.7 3.8
10
Ratio
A
B
C
−0.1 0.8 1.2
−0.3 0.8 1.4
−0.9 0.7 1.6
D
E
1.7 2.2
1.9 2.5
2.3 3.2
WA Tomorrow Population Report No. 7
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
Table 3: Population Forecasts by Age and Bands 2011
0 to 4
5 to 9
10 to 14
15 to 19
20 to 24
25 to 29
30 to 34
35 to 39
40 to 44
45 to 49
50 to 54
55 to 59
60 to 64
65 to 69
70 to 74
75 to 79
80 to 84
85 and over
A
240
240
260
190
70
70
110
170
220
280
310
340
370
300
200
130
75
65
B
280
280
300
230
130
150
180
230
280
320
340
350
390
310
210
130
80
70
C
310
300
320
250
160
190
230
270
310
340
350
370
400
320
210
140
80
70
D
340
330
340
270
190
240
270
310
350
370
370
380
400
320
220
140
80
70
E
370
360
370
300
240
290
330
360
390
400
390
390
420
330
220
140
85
75
Table 4: Population Forecasts by Age and Bands 2016
0 to 4
5 to 9
10 to 14
15 to 19
20 to 24
25 to 29
30 to 34
35 to 39
40 to 44
45 to 49
50 to 54
55 to 59
60 to 64
65 to 69
70 to 74
75 to 79
80 to 84
85 and over
WA Tomorrow Population Report No. 7
A
210
270
220
180
90
95
140
160
190
230
280
320
350
360
270
170
95
75
B
280
320
270
230
140
160
210
230
260
300
330
350
380
390
290
180
100
80
11
C
330
360
300
260
170
200
260
290
310
340
360
370
400
400
290
180
100
85
D
380
390
330
280
190
240
310
330
360
380
390
390
410
410
300
190
110
85
E
440
430
370
320
230
290
370
390
420
440
430
420
430
430
310
190
110
90
Forecast Profile
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
Table 5: Population Forecasts by Age and Bands 2021
0 to 4
5 to 9
10 to 14
15 to 19
20 to 24
25 to 29
30 to 34
35 to 39
40 to 44
45 to 49
50 to 54
55 to 59
60 to 64
65 to 69
70 to 74
75 to 79
80 to 84
85 and over
A
230
260
250
160
85
100
160
200
190
210
260
300
350
360
330
230
130
95
B
300
330
310
210
130
160
220
270
270
290
320
350
390
390
360
240
130
100
C
340
370
340
240
160
190
260
320
330
340
370
390
410
410
370
250
140
110
D
380
410
370
270
190
220
300
360
370
390
400
410
430
430
380
260
140
110
E
440
460
420
310
220
270
360
430
440
450
460
450
460
450
400
270
150
110
Table 6: Population Forecasts by Age and Bands 2026
0 to 4
5 to 9
10 to 14
15 to 19
20 to 24
25 to 29
30 to 34
35 to 39
40 to 44
45 to 49
50 to 54
55 to 59
60 to 64
65 to 69
70 to 74
75 to 79
80 to 84
85 and over
Forecast Profile
A
240
280
250
190
80
95
150
210
230
220
240
290
350
370
330
280
170
130
B
300
340
310
240
130
150
210
280
310
310
320
350
400
410
370
300
180
140
12
C
350
380
350
270
150
180
260
330
370
360
370
390
430
430
380
320
190
140
D
390
420
390
300
180
220
290
370
410
400
410
430
450
450
400
330
200
150
E
440
470
440
350
220
260
350
430
480
470
470
470
490
480
420
340
200
150
WA Tomorrow Population Report No. 7
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
Table 7: Boxplot Values for Top 20%
Lower Whisker
Box Bottom
Median
Top Box
Upper Whisker
Births Deaths Interstate Intrastate Overseas
1 400
-800
-300
-3 000
1 800
1 400
-800
-100
-1 800
2 700
1 500
-700
0
-1 400
3 100
1 600
-700
100
-1 000
3 800
1 800
-700
200
-300
5 500
Table 8: Boxplot Values for 60-80%
Lower Whisker
Box Bottom
Median
Top Box
Upper Whisker
Births Deaths Interstate Intrastate Overseas
1 200
-700
-300
-1 200
700
1 300
-700
-100
-500
1 400
1 300
-700
-100
-300
1 700
1 300
-700
0
-100
2 000
1 400
-700
200
400
2 800
Table 9: Boxplot Values for Middle 20%
Lower Whisker
Box Bottom
Median
Top Box
Upper Whisker
Births Deaths Interstate Intrastate Overseas
1 100
-700
-400
-300
-300
1 100
-700
-200
200
400
1 200
-700
-100
500
600
1 200
-700
0
700
900
1 300
-700
200
1 200
1 700
Table 10: Boxplot Values for Top 20-40%
Lower Whisker
Box Bottom
Median
Top Box
Upper Whisker
Births Deaths Interstate Intrastate Overseas
900
-700
-400
500
-2 000
900
-700
-200
1 100
-1 000
1 000
-700
-100
1 300
-600
1 000
-700
0
1 600
-200
1 100
-600
100
2 300
500
WA Tomorrow Population Report No. 7
13
Forecast Profile
Population forecasts for Bridgetown-Greenbushes (S) 2006 to 2026
Table 11: Boxplot Values for Bottom 20%
Lower Whisker
Box Bottom
Median
Top Box
Upper Whisker
Forecast Profile
Births Deaths Interstate Intrastate Overseas
300
-700
-400
1 600
-5 500
600
-700
-200
2 300
-3 400
700
-600
-200
2 700
-2 600
800
-600
-100
3 300
-2 000
900
-600
100
4 900
-1 100
14
WA Tomorrow Population Report No. 7