How should recruitment to public health surveys be conducted?

The aim of the public health survey in the Norwegian counties is to obtain information that is useful for public health work. In 2018, two parallel data collection processes were undertaken in Hordaland county. Both samples were drawn randomly from the National Population Register, but one of these was limited to users of the helsenorge.no website. The purpose of this article is to investigate the degree to which limiting users to the helsenorge.no website leads to selection bias beyond the selection that occurs through ordinary non-participation.

Services for Sensitive Data (TSD) was used in the data collection for the sample drawn from the National Population Register (n = 36 000), and the helsenorge.no platform was used in the data collection for the sample limited to users of helsenorge.no (n = 30 000). The response rate was 40.8 % and 41.5 %, respectively.

For some outcome measures, the differences between the two datasets were modest (gender distribution, age, education and health habits). For variables that were more directly related to health, the differences were greater. In the helsenorge.no sample a higher proportion reported generally poorer health (29.4 vs. 24.0 %), mental health problems (13.6 vs. 11.6 %), disability pension (10.5 vs. 7.8 %) and long-term illness (13.3 vs. 9.3 %). Analyses of subgroups showed more pronounced differences in the proportion with generally poorer health and mental health problems between those with low education in the helsenorge.no sample and the corresponding group in the sample from the National Population Register.

Systematic and pronounced differences between the samples show that limiting recruitment to users of helsenorge.no's services results in further selection problems.