It is internationally recognised that improvements in data systems are needed to support action and progress in Indigenous peoples’ health outcomes.(1, 2) This includes the need for longitudinal data on wellbeing measures that are defined by Indigenous people. These calls also recognise the importance of Indigenous peoples’ role in determining data processes, including the sampling approaches applied. Longitudinal (cohort) studies can and do contribute to the growing evidence base on Indigenous perspectives on health and wellbeing but to date Indigenous cohort studies in health have been small in scale or confined to geographic areas or language groups.
Postal surveys have been routinely used as a primary method of recruitment for large-scale national cohort studies and are a particularly efficient method for dispersed populations.(3) Yet, postal response rates are unknown for most Indigenous populations, because cohort studies in these populations have tended to opt for other recruitment approaches.(4–6) Examination of response rates is needed to determine if postal surveys are an effective mechanism for the collection of large scale, longitudinal data on Indigenous health.
In Australia, established Indigenous cohort studies (including: Aboriginal Birth Cohort Study,(7) Longitudinal Study of Indigenous Children,(8) Study of Environment on Aboriginal Resilience and Child Health,(9) Talking about the Smokes,(10) Antecedents of Renal Disease in Aboriginal Children, Next Generation Youth Wellbeing Study(11)) have used field recruitment to sample from known populations in particular urban, regional, and remote areas. These field-based sampling approaches use purposive or strata-sampling with local Aboriginal or Torres Strait Islander data collectors who are employed or supported by partner Indigenous services. The face-to-face approach facilitates multiple options for completion of a survey, including self-completion; completion through interview; and, use of interpreters.
Aboriginal and Torres Strait Islander people comprise 3% of the Australian population, with a substantial proportion (19%) living in remote areas compared with 2% of non-Indigenous Australians living remotely.(12) These factors increase the complexities and resources required to recruit a national, heterogeneous sample that reflects the cultural, linguistic and geographic diversity of the population. All the cited Indigenous cohort studies are small-scale (n < 3000) and most have focussed on linguistically or culturally specific communities or geographic regions, presenting a substantial gap in large-scale, national longitudinal approaches to data collection among Aboriginal and Torres Strait Islander Australians.
Australian cohort studies have demonstrated that Aboriginal and Torres Strait Islander people do respond to postal surveys, though not always to the same extent as the total Australian population. The 45 and Up Study, involving adults aged ≥ 45 years from New South Wales (NSW), recruited 1939 Aboriginal participants through postal surveys (0.7% of overall cohort population compared to 2.8% of the NSW population). The 45 and Up survey sampled from the Medicare Australia database, which provides close to complete coverage of the total Australian population.(13) The 1999 Australian Women’s Health Survey (AWHS) found the proportion of Aboriginal and Torres Strait Islander women recruited from the postal survey was similar to the total Indigenous population (2.5%, n = 900).(14) However, Aboriginal women living remotely were under-represented in the AWHS cohort, despite an over-sampling of remote areas.(15) These respective studies achieved an overall response rates between 18% and 44% in total sample population, but response rates specific to Indigenous participants were not calculated.(15, 16) In both studies, the Indigenous status of participants could only be identified post recruitment.
Until recently, it has been virtually impossible to determine a sampling frame to specifically distribute postal surveys to Aboriginal and Torres Strait Islander people. Telephone books and electoral rolls do not include Indigenous status information. Hospital records and other health registers do record Indigenous status, however data can: be unreliable due to poor recording of Indigenous status; be restricted due to privacy; be available only at state or territory-levels;(17), and include only the unwell population or birth population.(18–20) In 2006, Medicare added an Indigenous status question on enrolment and renewal forms.(21) This change has enabled researchers to request sampling of Indigenous persons from the Medicare Australia Enrolment Database. At October 2017, 533,832 people in the Medicare Australia Enrolment Database had indicated they were Aboriginal and/or Torres Strait Islander, covering ~ 60–70% of the total Indigenous population.
To our knowledge, Mayi Kuwayu, the National Study of Aboriginal and Torres Strait Islander Wellbeing is the first to test postal recruitment for an Indigenous cohort. The Study is a cohort of Aboriginal and Torres Strait Islander adults (≥ 16 years), designed to enable quantification of the relationships between health, culture and wellbeing.(6) The baseline sample is being recruited through a two-stage postal distribution: a preliminary distribution of 20,000 surveys and a main distribution of 180,000 surveys (total N = 200,000). The sample is supplemented with field recruitment. The aim of this cross-sectional analysis was to determine response rates to the preliminary postal distribution, overall and by age group, gender, level of remoteness and by jurisdiction, to inform the Study’s ongoing recruitment approaches.