The objective of this article is to understand how the change of mode from telephone to web affects data quality in terms of sample representation and substantive variable bias. To this end, an experiment, consisting of a web survey with and without a prepaid incentive, was conducted alongside the telephone Swiss election survey. All three designs used identical questionnaires and probability samples drawn from a national register of individuals.
First, our findings show that differences in completion rates mostly reflect different levels of coverage in the two modes. Second, incentives in the web survey strongly increase completion rates of all person groups, with the exception of people without Internet access or limited computer literacy. Third, we find voting behavior to be much closer to official figures in the web with the incentive version compared to the two other designs. However, this is partly due to the different sociodemographic compositions of the samples. Other substantive results suggest that the incentive version includes harder-to-reach respondents. Unit costs are much lower in the two web designs compared to the telephone, including when a relatively high incentive is used. We conclude that in countries with high Internet penetration rates such as Switzerland, web surveys are already likely to be highly competitive.
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