The effects of survey mode and sampling in Belgian election studies: a comparison of a national probability face-to-face survey and a nonprobability Internet survey


National probability election surveys are more and more abandoned. Decreasing response rates and the escalating costs of face-to-face and telephone interviews have strengthened election scholars’ reliance on nonprobability internet samples to conduct election surveys online. In a number of countries, experiments with alternative ways of recruiting respondents and different interview modes have been well documented. For other countries, however, substantially less is known about the consequences of relying on nonprobability internet panels. In this paper, we investigate the effects of survey mode and sampling method in the Belgian context. This is a particularly important and relevant case study because election researchers in Belgium can draw a sample of voters directly from the National Register. In line with previous studies, we find important differences in the marginal distributions of variables measured in the two surveys. When considering vote choice models and the inferences that scholars would draw, in contrast, we find minor differences.

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Fig. 2


  1. 1.

    Although it is important to point out that self-administered surveys come with problems as well, such as challenges related to measuring political knowledge in a reliable way (and preventing cheating) (Motta et al. 2016).

  2. 2.

    For the British Election Studies, the response rate has dropped from 79% in 1963 (Crewe et al. 1977) to 56% in 2015 (Fieldhouse et al. 2016); in Canada, the response rate has dropped from 63% in 1965 (Converse et al. 2002) to 37% in 2015 (Northrup 2016); in Australia, the response rate has declined from 63% in 1987 to 23% in 2015 (Cameron and McAllister 2016).

  3. 3.

    i.e., Canada, Germany, Great Britain, and the United States. See also Appendix A.

  4. 4.

    According to the United Nations, by 2014, 85% of Belgians were internet users. This is substantially higher than what was seen 5 or 10 years before (70 or 54% respectively). For more information, see

  5. 5.

    PartiRep stands for ‘Participation and Representation,’ an Inter-University Attraction Pole that was funded by the Belgian Science Policy. More information on the project can be found at

  6. 6.

    More info on this research project can be found at

  7. 7.

    Ansolabehere and Schaffner (2014) include a mail survey in their comparison as well. For a full overview of the exact sampling and mode comparisons in the studies cited in this literature review, see the supplementary materials.

  8. 8.

    Quotas for age were based on three broad age ranges: 18–34, 35–54, and 55–99 years. The education quotas as well were based on three categories: lower secondary education, upper secondary education, and tertiary education.

  9. 9.

    For the Flemish region: GMI, HPOl. and Toluna. In Wallonia: GMI, HPOL, Toluna, and SSI.

  10. 10.

    Passwords were sent to panelists along with the URL to the survey. In this way, access to the survey was controlled, and only the selected panelists could participate.

  11. 11.

    This was the case for the U.S.C. Dornsife/Los Angeles Times Daybreak poll during the 2016 presidential elections in the United States, cf.

  12. 12.

    We apply the FINALweightg weight for the PartiRep dataset and the WEIGHT1 weights for the MEDW data. For the pre-electoral wave, the PartiRep weight varies between 0.55 and 3.19, with a mean of 1.00 and a standard deviation of 0.43. The MEDW basic sociodemographic wave for the pre-electoral wave has a minimum value of 0.54 and a maximum value of 3.71. Its mean is 1.00 with a standard deviation of 0.43.

  13. 13.

    We compare the reported votes in the samples with the vote shares that parties obtained in the Flemish region, excluding voters in Brussels.

  14. 14.

    We do not estimate models explaining the vote for some of the smaller parties, as the datasets only included small numbers of respondents who voted for green, populist radical right, and extreme-left parties. It is important to note that Fig. 2 indicated that differences in the reported vote choices are somewhat larger for radical parties. As a result, not investigating the determinants of voting for these parties might lead us to underestimate differences between the two surveys.

  15. 15.

    When estimating the bivariate models without applying the sociodemographic weight, six interaction terms are significant (results available from the authors). Weighting thus only has a marginal impact when focusing on explanatory models.

  16. 16.

    For a desired significance level of 0.05, we divide 0.05 by the number of tests. In this case, 0.05/28 results in a p value threshold of 0.002 (Gelman et al. 2012).

  17. 17.

    Compared to a series of bivariate models on unweighted data, there is no improvement in the number of significant interaction terms. (Results available from the authors.)

  18. 18.

    For a 0.05-level and 21 tests, the p value threshold is 0.05/21, or 0.002.

  19. 19.

    In a supplementary analysis, reported in Appendix F, we have also verified whether the impact of political interest on vote choice differs in the two samples. For none of the seven parties (four parties in the Flemish region and three parties in the Walloon region), the interaction term—survey × political interest—is significant at conventional levels.

  20. 20.

    For, the Walloon sample as well Kolmogorov–Smirnov tests indicate that the distributions of respondents’ answers on the party like/dislike scales differ significantly between the two samples—without a single exception.

  21. 21.

    Unfortunately, we do not dispose of a large number of alternative dependent variables for which we could investigate whether the two surveys would lead to different conclusions regarding what explains it. Given the overall high turnout, and overreported turnout, explaining reported turnout is not an option. We did pursue an additional analysis to explain hypothetical turnout under voluntary voting rules. These supplementary analyses did not reveal strong differences between the two surveys that could not be attributed to wording differences.

  22. 22.

    However, it also has to be noted that the face-to-face interview can be much longer than the standard online survey. For that exact reason, it is not straightforward to compare the cost by respondent of the PartiRep survey to the cost of the MEDW survey.


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A previous version of this paper was presented during the Making Electoral Democracy Work mini-conference at the 113th Annual Meeting of the American Political Science Meeting, San Francisco, August 31-September 3, 2017. We thank Filip Kostelka for providing technical information on the MEDW-survey and Fernando Feitosa for research assistance. We are grateful to Shane Singh and Dieter Stiers for commenting on previous drafts of the papers and the anonymous reviewers of this journal for excellent suggestions.

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Correspondence to Ruth Dassonneville.

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Dassonneville, R., Blais, A., Hooghe, M. et al. The effects of survey mode and sampling in Belgian election studies: a comparison of a national probability face-to-face survey and a nonprobability Internet survey. Acta Polit 55, 175–198 (2020).

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  • Election study
  • Belgium
  • Survey mode effects
  • Representativeness
  • Nonprobability sample