The HILDA Sample

Sample Design

Overview

In line with leading panel studies conducted in other countries, the sampling unit is the household, and members of those households will be traced over an indefinite life. The wave 1 sample is then automatically extended over time by following rules that add to the sample:

• any children born to or adopted by members of the selected households; and
• new household members resulting from changes in the composition of the original households.³⁹

These following rules, in combination with the initial sample that is intended to be representative of all Australian households, provide a mechanism for ensuring that the panel retains its cross-sectional representativeness over time.

While all members of the selected households are defined as members of the sample, individual interviews are only conducted with those aged 15 years and over. Some limited information about people under 15, however, is collected from an appropriate adult member of the household.⁴⁰

Reference Population

The reference population for wave 1 was all members of private dwellings in Australia , with the following exceptions:

• certain diplomatic personnel of overseas governments, customarily excluded from censuses and surveys;
• overseas residents in Australia (that is, persons who had stayed or intended to stay in Australia less than one year);
• members of non–Australian defence forces (and their dependents) stationed in Australia
• residents of institutions (such as hospitals and other health care institutions, military and police installations, correctional and penal institutions, convents and monasteries) and other non-private dwellings (such as hotels and motels); and
• people living in remote and sparsely populated areas.

Further, to ensure that all members of the in-scope population have the same probability of selection, dwellings that were not primary places of residence (for example, holiday homes) were also excluded.

These coverage rules are broadly in line with those adopted by the Australian Bureau of Statistics (ABS) in the monthly Labour Force Survey supplements.⁴¹ There are, however, two major differences. First, unlike the ABS, individuals at boarding schools, halls of residence and university colleges were included in the reference population for wave 1. Second, and again in contrast to ABS practice, military personnel who reside in private dwellings are part of the reference population for wave 1.

Note that while all members of the selected households are defined as members of the sample, individual interviews were only conducted with those aged 15 years and over.

Sampling Units

The sampling unit is the household defined, following the ABS, as ‘a group of people who usually reside and eat together’.⁴² The ABS clarifies how this definition is operationalised. Specifically, a household is either:

• a one-person household, that is, a person who makes provision for his or her own food or other essentials for living without combining with any other person to form part of a multi-person household; or
• a multi–person household, that is, a group of two or more persons, living within the same dwelling, who make common provision for food or other essentials for living. The persons in the group may pool their incomes and have a common budget to a greater or lesser extent; they may be related or unrelated persons, or a combination of both.

In general, persons who live in more than one household were only treated as members of the household where they spent most of their time. People who lived in another private dwelling for more than 50 per cent of the time were not treated as part of the household. Visitors to the household were also not treated as part of the household. Finally, people who usually lived in the household but were temporarily absent for work, school or other purposes were treated as part of the household, and this meant that a small proportion of interviews were conducted in locations other than at the household address.

Note again that we varied from the ABS practice in how we treat children attending boarding schools and halls of residence while studying. Specifically, while these dwellings are out of scope in wave 1, such individuals were treated as members of sampled households provided they spent at least part of the year in the sampled dwelling.

Sample Selection

The households were selected using a multi-staged approach. First, a sample of 488 Census Collection Districts (CDs) were selected from across Australia (each of which consists of approximately 200 to 250 households). Second, within each of these CDs, a sample of 22 to 34 dwellings was selected, depending on the expected response and occupancy rates of the area. The selections were made after all dwellings within each of the CDs were fully listed. Finally, within each dwelling, up to three households were selected to be part of the sample.

Watson and Wooden (2002b) provides further details of the sampling methodology.

Following Rules

The fully and partially responding households in wave 1 form the basis of the indefinite life panel. Members of these households are followed over time and the sample is extended to include:

• any children born to or adopted by members of the selected households; and
• new household members resulting from changes in the composition of the original households.

Continuing Sample Members (CSMs) include all members of wave 1 households (including children). Any children born to or adopted by CSMs are also classified as CSMs. Further, all new entrants to a household who have a child with a CSM are converted to CSM status. CSMs remain in the sample indefinitely. All other people who share a household with a CSM in wave 2 or later are considered Temporary Sample Members (TSMs).

Where the household has moved, split or moved and split, the interviewers and office staff track the CSMs. The CSMs (along with their new household) are then interviewed, where applicable, at their new address or by phone.⁴³ TSMs that split from a household and are no longer part of a household with a CSM are not followed. However, if the TSM is converted to a CSM, then they are followed for interview as any CSM would be.

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