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INTRODUCTION
Limitations of census data for preparing projections 15 The census question on relationships in the household collects information on only one level of household relationship, that of each person in the household to Person 1 and/or Person 2 on the census form. In situations where household members are related to each other but not to Person 1 or Person 2, then that second level of relationships may be lost. This may lead to a distortion and underestimation of the number of families, as well as relationships within the household and family type. 16 Person level data within the Census is based on place of enumeration. Although responses to the census question on persons temporarily absent from the household gives person level data for absent usual residents, this is categorised by three broad age groups only. These persons cannot be allocated to household, family and relationship in household types, by fiveyear age groups. Visitors to households on census night were excluded from the propensities because they are not included in family coding in the Census. Estimated Resident Population 17 The ABS publishes quarterly estimates of the ERP at the national and State level, and annually at the Statistical Local Area (SLA) level. The ERP is based on census counts of usual residents. Account is taken of census underenumeration and the number of Australian residents temporarily overseas at the time of the census. 18 ERP is obtained by adding to the estimated population at the beginning of each period the components of natural increase (on a usual residence basis) and net overseas migration. For States and Territories, account is also taken of estimated interstate movements involving a change of usual residence. After each census, estimates for the preceding intercensal period are revised by incorporating an additional quarterly adjustment (intercensal discrepancy) to ensure that the total intercensal increase agrees with the difference between the estimated resident populations at the two respective census dates. 19 The ERP does not distinguish between persons resident in private and NPDs. Family and household projections are concerned exclusively with the population usually resident in private dwellings. Therefore, for the purposes of these projections, the proportion of the census count in NPDs is identified and excluded from the calculation of numbers of families and households but they are included in the analysis of living arrangements. Population Projections 20 The ABS publishes population projections every two to three years. See Population Projections, 19972051 (Cat. no. 3222.0.) Projections of the population by age and sex are produced for Australia, the States and Territories and capital city/balance of State, excluding Australian Capital Territory balance of State. The projections are based on a combination of assumptions on future levels of births, deaths and migration to arrive at the size, structure and distribution of Australia's population into the next century. Three main series are published. 21 Each of the household, family and living arrangement projections use Series K population projections. This ensures that differences in the household and family projections series reflect changes in family and household structure. The Series K population projection was chosen as it is most closely aligned with current trends. Series K assumes high levels of net overseas migration (annual net overseas migration gain of 90,000), medium interstate migration, low fertility (total fertility rate declines to 1.6 births per woman by 200506), and declining mortality. METHOD Overview and assumptions 22 There are a number of techniques which may be used for projecting household and family numbers, including both static and dynamic models. The ABS uses a propensity method which identifies propensities from the Census of Population and Housing for people in each fiveyear age group to be living in different living arrangement types. Trends observed in the propensities over the last three censuses are then projected forward and applied to the projected population. Projected numbers of households and families are derived from the projected living arrangements of the population. 23 A household is defined as a group of two or more related or unrelated people who usually reside in the same dwelling, who regard themselves as a household and who make common provision for food or other essentials for living; or a person living in a dwelling who makes provision for his or her own food and other essentials for living, without combining with any other person. The Census categorises households as discrete units, thus the projection method is based on the assumption that households are discrete units. It is acknowledged that some living arrangements are fluid and that because of study, work or shared care arrangements a family member may be associated with more than one household. For example, fulltime students living away from home may still be dependent on their family household. 24 This method assumes that trends observed over three censuses accurately reflect underlying changes in living arrangement characteristics over that time period. 25 Living arrangement propensities were calculated using a large proportion (94% in 1996) of the census count of Australian usual residents. People not counted in the census, or not included in the classification of living arrangements, but included in the projected population were proportionally distributed across the living arrangement types. This assumes that these people have the same living arrangement distribution as those included in the calculation of the propensities. 26 Distributing by this method is likely to lead to an underestimation of lone person households. If an entire household is absent on Census night, that household can be missed. However, if some members of the household are at home, the household type can be coded based on the information of those members remaining at home, and the information provided for members temporarily absent. It is therefore more likely for households to be missed if the household consists of only one member. Step 1: Producing the population distribution by living arrangement 27 Using the 1986, 1991 and 1996 Census datasets, census counts were classified by living arrangement, and fiveyear age groups. Agesex specific propensities were used for lone parent and lone person living arrangements. For other living arrangements only agespecific propensities were used to ensure that the propensities would be robust, particularly for the smaller balance of States. Visitors to households, overseas visitors and people in nonclassifiable households were excluded. Step 2: Calculation of the propensities 28 From the distribution produced at Step 1, the propensity of persons to be in different living arrangements, by fiveyear age groups was determined. The propensity of each age group to live in each living arrangement was calculated using the total count of persons by fiveyear age group as the denominator (excluding visitors to households, overseas visitors and people in nonclassifiable households). For example, in 1996 there was a 85.3% likelihood that a 04 year old would be a child in a couple family, and a 4.5% likelihood that a 2529 year old would be a female lone parent. The propensities for 1986, 1991 and 1996 are shown in the Appendix. Step 3: Projection of the propensities 29 Each series of household and family projections is based on assumptions about future rates of change in propensities. These rates of change are determined from trends in propensities over the last three censuses (see Appendix). For the purpose of trend analysis, the 1986 and 1991 living arrangement counts have been recoded to achieve consistency with the 1996 definitions (See Explanatory Notes 1215). 30 A line of best fit was calculated for each living arrangement type and age group, using the observed 1986, 1991 and 1996 propensities. The annual rate of change for each line of best fit was calculated. For each of the three series (see Chapter 2, Assumptions), the calculated rates of change were applied to the observed 1996 propensities, by age group and living arrangement type, at the fractions specified in the assumptions (zero, reducing from one and one, respectively). If through the projection process any propensity became negative, it was constrained to zero and held constant at zero from that point onwards. Projected propensities were subsequently adjusted to add to 100% in each five year age group. Step 4: Application of the propensities to the projected population 31 The propensities calculated at Step 3 were applied to the ERP in the base year and the projected population in subsequent years, by fiveyear age group. Multiplying the propensity to live in each living arrangement for a given fiveyear age group, by the projected population for that fiveyear age group, gives the distribution of the population for the projected year, by living arrangement. Step 5: Forcing of subState and State totals to Australian totals 32 Propensities were produced for Australia, each State and Territory and each balance of State (excluding Australian Capital Territory balance of State) and applied to the projected population. As the propensities were projected independently at each geographical level, once they were applied to the projected population their sum did not equal the distribution of living arrangements for Australia. In order to ensure consistency in the produced number of persons, constraints were applied to person level data to ensure that the sum of the subStates and States add to that of Australia. 33 Forcing was conducted using twodimensional iterative proportional fitting (IPF). To constrain to Australian totals, a matrix was constructed for person counts for each fiveyear age group for each State/Territory and living arrangement type. The figures in the body of the matrix were proportionally scaled across rows and columns simultaneously constraining to the Australian living arrangement totals in the final column and State age totals in the final row. The scaling process was repeated several times until stability was attained within the matrix. Decimals produced by the scaling process were then rounded, with column and row totals preserved. The final matrix was fed back into the extrapolation process, forming the base for the next year's projections. 34 This procedure was then conducted at the subState level. The same process was followed, with the matrix constrained simultaneously to State or Territory living arrangement totals and substate age totals. For a more detailed description of the IPF procedure, see Purcell, N.J. and Kish, L. (1979) Estimations for small domains, Biometrics, 35, pp. 365384. Step 6: Calculating the number of families and households 35 For each projected year, family and household numbers are derived from the projected living arrangements. This involved a number of sequential steps: Step 6.1 Deriving numbers of families from the living arrangement types 36 The number of families are derived from the living arrangements of the projected population. The number of couple families (with or without children) is half the number of partners in couple families (with or without children). The number of oneparent families is the number of male lone parents plus the number of female lone parents. The number of 'other families' is calculated by dividing the number of related individuals in other families by the average size of this family type at the 1996 Census (2.1259). Step 6.2 Converting families to family households 37 Family households can contain more than one family. In order to produce numbers of households, families were converted to households using a ratio calculated from the 1996 Census (ratio of 0.9843). Step 6.3 Deriving numbers of nonfamily households 38 Numbers of nonfamily households were also derived from the living arrangement types. The number of lone person households was calculated as a sum of the number of male lone persons plus the number of female lone persons. The average size of group households at the 1996 Census was calculated by dividing the number of group household members by the number of group households. This figure (2.278889 persons per household) was then used to calculate the number of group households in the projections. Step 6.4 Deriving the total number of households 39 Family and nonfamily households were added to produce the total number of households. DIFFERENCE FROM ABS HOUSEHOLD ESTIMATES 40 It should be noted that the proposed method produces household numbers that are different to the ABS household estimates in the base year (1996) and subsequent years. For the base year this is a result of the use of different methods for estimating and projecting household numbers. In subsequent years the discrepancy is a result of the base year inconsistency and the inherent differences between estimates and projections. As the method for estimating households is currently under review and as changes are likely to be made to ensure more stability in the estimates, the inconsistency between the number of households estimated by the two methods has been allowed, both for 1996 and for subsequent years. In the base year the method used in these projections gives results which are 1.5% higher than household estimates. GEOGRAPHICAL AREAS 41 The propensities were derived using the area boundaries as they existed at the relevant reference points (1986, 1991 and 1996). It is considered that the use of such propensities (rather than based on revised areas) would not have had a significant effect on the comparability of the propensities over time. ROUNDING 42 In this publication figures of less than one million are rounded in the text to the nearest thousand and figures of more than one million are rounded to the nearest one hundred thousand. In the tables figures are rounded to the nearest hundred. COMPARISON WITH STATE GOVERNMENT HOUSEHOLD PROJECTIONS 43 The following table has been provided for comparative purposes. It gives details of ABS and State and Territory government household projections at the capital city and balance of State levels for 2011. It should be noted that the ABS projections are 1996based while, with the exception of Victoria, the State and Territory projections are 1991based. Not all State and Territory governments produce household projections.
ACKNOWLEDGMENT 44 ABS publications draw extensively on information provided freely by individuals, businesses, governments and other organisations. Their continued cooperation is very much appreciated: without it, the wide range of statistics published by the ABS would not be available. Information received by the ABS is treated in strict confidence as required by the Census and Statistics Act 1905. RELATED PUBLICATIONS 45 Users may also wish to refer to the following ABS publications:
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