TECHNICAL FACT SHEET – OCTOBER 2014
NATIONAL VISITOR SURVEY DUAL FRAME SAMPLE
SAMPLE DESIGN
Two sample frames have been used for the National Visitor Survey (NVS) since January 2, 2014:

residential fixed line frame, which has been used for the NVS since its commencement in 1998

mobile phone frame, added to the NVS in 2014 in order to provide coverage of the mobile only population,
which is not covered by fixed line frames.
Both of these frames are generated by a Random Digit Dialling (RDD) process that enables these two frames to fully
cover all residential phone numbers and all mobile numbers.
The two sample frames are combined for the NVS using an ‘overlapping’ dual frame sample design. In this design,
shown in the diagram below, respondents who have both a landline and a mobile phone are eligible for the NVS
irrespective of the sample frame from which they were selected. This is in contrast to a ‘non-overlapping’ dual frame
design, in which respondents with both a landline and a mobile phone would only be eligible if they were selected from
one of these frames and would not be eligible if selected from the other frame.
Overlapping dual frame sample design
The NVS employs an overlapping dual frame sample design, as studies have shown that it generally provides better
coverage of the population with both a fixed line and mobile phone (sometimes called the dual phone population).
Special weighting measures are required with overlapping dual frame sample designs to account for the fact that the
population covered by the overlap of the two frames has higher probabilities of selection than the population not in the
overlap. This is because members of the overlap population can be selected from both sample frames, whereas those
not in the overlap can only be selected from one sample frame.
In the case of the NVS, the dual phone population has a higher probability of selection than the mobile only or fixed
line only populations. Unless a special form of weighting is used to account for these probabilities of selection, the
survey estimates are subject to bias.
WEIGHTING
Weighting enables survey results to be projected to the NVS target population (Australian residents aged 15 years
and over), and needs to account for the differing probabilities of sample selection from dual phone, fixed line only and
mobile only populations, along with other aspects. The following lists sample design factors which create differing
probabilities of selection:
Fixed line sample
1. Differing probabilities of selection are created from the selection of a non-proportional sample across the 13
NVS fixed line strata
2. The selection of a household from a sample of fixed line phones means that each fixed line respondent is
selected with probability proportional to the number of landlines owned by a respondent’s household
3. The selection of one random respondent per household creates probabilities of selection which are inversely
proportional to the number of in-scope residents (residents aged 15+) of the respondent’s household
4. The overlapping dual frame design creates differing selection probabilities for dual phone and fixed line only
respondents
Mobile sample
5. The selection of a person from a sample of mobile phones means that each mobile phone respondent is
selected with probability proportional to the number of working mobile phones owned by that respondent
6. The overlapping dual frame design creates differing selection probabilities for dual phone and mobile only
respondents
Both fixed line and mobile sample
7. The method of recall used in the NVS questionnaire creates different probabilities of selection across the days
in a given reporting period.
The NVS weighting process takes account of all the above factors, with the exception of factor 21.
Another weighting process is also used for the NVS to account for the differential response rates across the sample.
In this process, the sample is grouped into weighting cells which are defined by:
Age
x gender
x survey strata (the 13 geographic strata of the NVS)
x phone ownership group (mobile only, fixed line only and dual phone)
x sample source (fixed line or mobile)
In this final weighting stage, the estimated current population sizes of these groups are used in conjunction with the
respective selection probabilities. The estimated population sizes are based on the Australian Bureau of Statistics’
Estimated Resident Population (ERP) counts for the relevant period, as well as commercially available survey
information which provides estimates of the phone ownership group sizes classified by ‘age x gender x strata’.
1
Previous studies show that there was negligible benefit to the NVS from this form of weighting