International health policy survey in 11 countries: assessment of non-response bias in the Norwegian sample

BACKGROUND: International health policy surveys are used to compare and evaluate health system performance, but little is known about the effects of non-response. The objective of this study was to assess the effects of non-response in the Norwegian part of the Commonwealth Fund international health policy survey in 2009. METHODS: As part of an international health policy survey in 2009 a cross-sectional survey was conducted in Norway among a representative sample of Norwegian general practitioners. 1 400 randomly selected GPs were sent a postal questionnaire including questions about the Norwegian health care system, the quality of the GPs' own practice and the cooperation with specialist health care. The survey included three postal reminders and a telephone follow-up of postal non-respondents. The main outcome measures were increase in response rate for each reminder, the effects of demographic and practice variables on response, the effects of non-response on survey estimates, and the cost-effectiveness of each reminder. RESULTS: After three postal reminders and one telephone follow-up, the response rate was 59.1%. Statistically significant differences between respondents and non-respondents were found for three variables; group vs. solo practice (p = 0.01), being a specialist or not (p < 0.001) and municipality centrality (least central vs. most central, p = 0.03). However, demographic and practice variables had little association with five outcome variables and the overall survey estimates changed little with additional reminders. In addition, the cost-effectiveness of the final reminders was poor. CONCLUSIONS: The response rate in the Norwegian survey was satisfactory, and the effect of non-response was small indicating adequate representativeness. The cost-effectiveness of the final reminders was poor. The Norwegian findings strengthen the international project, but restrictions in generalizability warrant further study in other countries.