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INDEPENDENCE FROM THE CENSUS

One of the purposes of the PES is to provide an independent check on Census coverage. There are two aspects to this independence: population independence and operational independence.

Population independence means that there should be no subgroups of the population where being missed in the Census indicates that a person or dwelling is more likely to be missed by the PES also. This is harder to achieve than operational independence, though the PES estimation process can adjust for this, to some extent, by subdividing the population into smaller groups where the assumption of population independence is more likely to be true.

Operational independence requires that Census operations do not influence the PES in any way, and vice versa. Steps are taken to maintain the operational independence of the PES from the Census at every stage of the survey, including enumeration, processing and administration. These steps include:

• selecting the PES sample from an independent sample frame;
• using separate office staff in the PES and Census where possible;
• ensuring the PES interviewers are not employed as Census field staff in the same area, and vice versa; and
• maintaining the confidentiality of the PES sample so that Census field and office staff are not aware which areas are included in the PES.

Householders are asked the PES questions face-to-face by experienced, highly trained interviewers, whereas most Census forms are self-completed. The PES is also a much smaller scale operation (and hence easier to control) than the Census. These features enable the PES to deliver an accurate estimate of the percentage of people and dwellings missed by the Census.

The Census can also be used to form an estimate of the percentage of dwellings and people missed by the PES. The PES excludes non-private dwellings (hotels, motels, hospitals) for operational reasons. The PES is also conducted several weeks after the Census, so the respondent's recollection of their location on Census night may not be entirely accurate. Census has special procedures for enumerating homeless people, while the PES is essentially a survey of dwellings and the people who reside in them. Thus the Census may include some dwellings and people that the PES misses. PES estimation implicitly accounts for the dwellings and people missed in the PES but counted in the Census.

Late census forms

For some people who have not returned a Census form, contact from the ABS following selection in the PES acts as a reminder and possible motivator to return a completed Census form. These late returns, if not identified, would result in the PES sample having a higher proportion of Census response than in the overall population. To protect against this, all Census forms received after the start of PES field work are deemed 'late' and treated differently in PES estimation.

Correlation bias

On the assumption that the Census and the PES are independent, the estimate of the percentage missed by the PES but found by the Census, and the percentage missed by the Census but found by the PES, can be used to construct estimates of the percentage missed by both PES and Census.

Despite efforts to maintain independence, the likelihood of a person being missed in the PES may be related to whether they were missed in the Census. This may result in a 'correlation bias' in the PES estimates. To minimise this bias, PES estimation takes account of the fact that different groups have a different likelihood of being missed.