Web Survey Bibliography
In order to investigate the advantage of mixed-mode surveys, selection effects between the modes should be evaluated. Selection effects refer to differences in respondent compositions on the target variables between the modes. However, estimation of selection effects is not an easy task because they may be completely confounded with measurement effects between the modes (differences in measurement error). Publications concerning the estimation of these mode effects are scarce. This paper provides and compares three methods that enable evaluating measurement effects and selection effects separately. The first method starts from existing publications where the confounding problem is usually avoided by introducing a set of mode-insensitive variables into the analysis model. However, this paper will show that this method precludes exact estimation of the effects. The second and the third method make use of a mixed\-/mode sample extended by comparable single-mode data. The advantages, assumptions and disadvantages of all three methods are discussed. Each method will further be illustrated on a set of six variables about the opinion about surveys of the Flemish population. The results show large differences between the methods.