This paper focuses on mode and incentive effects on substantive variables as well as associations between these variables. We use data from the 2011 Swiss Electoral Study (Selects), which included an additional web experiment conducted using the same questionnaire and probability sampling frame as the usual telephone survey. This allows us to overcome limitations of previous similar comparisons, which mostly used opt-in panels. As the web survey was separated into incentive and control groups, we can also study ways to improve the engagement of participants in internet surveys.
Theory and previous research lets us expect disparities in point estimates due to differences related to coverage, nonresponse, and measurement error in the three scenarios. It is possible to partially control for coverage error using sociodemographic information from the sampling frame and for selection effects by comparing the actual vote choice and turnout figures from the election to reported behaviour - a specific benefit of electoral studies. Results from univariate analyses show that the regular internet survey produces results very close to the telephone survey, while the incentivized internet survey differs more often from the two other samples. However, the latter yields figures that are closer to actual election results, even after controlling for sample composition. Relationships between variables are tested using regression models, which show few differences between the three conditions. In general, the results speak in favour of the incentivized internet survey. However, care must be taken when comparability of time series is an important factor.