Australian Bureau of Statistics
Celebrating the International Year of Statistics 2013

SAMPLE DESIGN AND SELECTION

Sample design

The 2011-12 NHS was conducted using a stratified multistage area sample of private dwellings. Decisions on the appropriate sample size, distribution and method of selection rested on consideration of the aims of the survey, the topics it contained, the level of disaggregation and accuracy at which the survey estimates were required, and the costs and operational constraints of conducting the survey. The sample was designed to provide:

• relatively detailed estimates for each state/territory and Australia;
• relatively detailed estimates for Capital City/Balance of State areas within each state and territory;
• broad level estimates for regions within larger states and territories; and
• estimates for those characteristics which are relatively common, and sub-populations which are relatively large and spread fairly evenly geographically.

To achieve these design objectives, state and territory sampling fractions were set as shown in the following table, which also depicts the corresponding expected number of fully responding households. The sample selection procedures described below result in every dwelling in the same state or territory having a known probability of selection, equal to the state or territory sample fraction.

	NSW	Vic.	Qld	SA	WA	Tas.	NT	ACT	Aust.
Approximate sampling fractions	1/1003	1/854	1/702	1/339	1/413	1/142	1/62	1/105	1/551
Expected fully responding households	2 800	2 640	2 530	2 000	2 100	1 470	1 150	1 310	16 000

Actual numbers of fully responding households are available in the Response rates section of this chapter.

Within selected dwellings, a random sub-sample of residents was selected as follows:

• one adult (aged 18 years and older); and
• one child aged 0-17 years (where applicable).

Sample selection

The area-based selection ensures that all sections of the population living in private dwellings within the geographic scope of the survey were represented by the sample. Each state and territory was divided into geographically contiguous areas called strata. Strata are formed by initially dividing Australia into regions, which are formed within state/territory boundaries, and which basically correspond to the Statistical Division or Statistical Subdivision levels of the Australian Standard Geographical Classification (ASGC) (cat. no.1216.0). Each stratum contains a number of Census Collection Districts (CDs), which on average contain around 250 dwellings.

In capital cities and other major urban or high population density areas, the dwelling sample was selected in three stages:

• a systematic sample of CDs was selected from each stratum with probability proportional to the number of dwellings in each CD;
• each selected CD was divided into groups of dwellings or blocks of similar size, and one block was selected from each CD, with probability proportional to the number of dwellings in the block; and,
• within each selected block a list of all private dwellings was prepared and a systematic random sample of dwellings was selected.

In Hobart, parts of Darwin and some strata of high population growth, the CD stage of selection was omitted leaving only two stages of selection.

In strata with low population density each stratum was initially divided into units, usually corresponding to towns or Statistical Local Areas (SLAs), or combinations of both, and one or two units were selected from each stratum. Within selected units, the sample of dwellings was arrived at in the same manner as outlined for high population density areas.

In the NHS a total sample of approximately 21,100 households was selected which, taking account of an expected rate of sample loss (e.g. vacant dwellings, dwellings under construction, etc.) of 14% and an expected rate of non-response of 10%, was designed to achieve the desired sample of around 16,000 fully responding households.

To take account of possible seasonal effects on health characteristics (including nutrition and physical activity), the NHS and NNPAS were enumerated over 12-month periods. NHS was enumerated from 6 March 2011 to 17 March 2012. Collection Districts were allocated randomly over four sub-periods:

• March 2011 to May 2011;
• June 2011 to August 2011;
• September 2011 to November 2011; and
• December 2011 to March 2012.

A pause in enumeration occurred between 6 August 2011 and 2 October 2011, to facilitate field work associated with the 2011 Census of Population and Housing.

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