Inclusion of mobile telephone numbers into an ongoing population health survey in New South Wales, Australia using an overlapping dual -frame design: final weighting strategy

Background In 2012 mobile telephone numbers were included into the ongoing NSW Population Health Survey (NSWPHS) using an overlapping dual-frame design. Previously in the NSWPHS the sample was selected using random digit dialling (RDD) of landline telephone numbers. The survey was undertaken using computer assisted telephone interviewing (CATI). Details about the methodology, call outcomes and representativeness of the sample in the first quarter of 2012 are published elsewhere. The weighting strategy also needed to be revised to manage the differing probabilities of selection by frame and to adjust for the increased chance of selection of dual-phone users. This paper describes and details the final weighting strategy adopted to properly combine the data from the two overlapping sample frames in the NSWPHS and the benchmark populations used, based on the limited information available in Australia. Methods/Design Estimates of the number of telephone numbers for the landline and mobile phone frames, used to calculate the differing probabilities of selection by frame, for NSW and by stratum, were obtained by apportioning Australian estimates as none were available for NSW. The weighting strategy was then developed by calculating person selection probabilities, selection weights, applying a constant composite factor to the dual-phone users sample weights, and benchmarking to the latest NSW population by age group, sex and stratum. Conclusions The inclusion of mobile telephone numbers, through an overlapping dual-frame design, improved the coverage of the survey and an appropriate weighing procedure is feasible, although it added substantially to the complexity of the weighting strategy. Access to accurate Australian, State and Territory estimates of the number of landline and mobile telephone numbers and type of phone use by at least age group and sex would greatly assist in the weighting of dual-frame surveys in Australia.