Mixing data collection modes provides an opportunity to compensate for the weaknesses of each individual mode. Face-to-face interviewing may be the best mode to keep the survey error as small as possible; it is however, the most expensive. Web interviewing is much cheaper, but leads to under coverage. Mixing modes may however reduce data comparability since different modes a) provide access to different types of people, b) attract different types of respondents, and c) elicit different responses. The decision to mix modes requires survey practitioners to evaluate and quantify the impact of mode on data quality. This study explores some of the issues surrounding the use of CAWI-Computer-assisted web-based interview surveys, in particular the extent to which data from a CAWI survey can be matched to data from a CAPI-Computer-assisted personal interview. Some hypotheses about what causes differences in data from web-based surveys and face-to-face surveys are discussed. These include i) interviewer effect and social desirability bias in face-to-face methodologies, ii) the mode effects of web-based and face-to-face survey methodologies, including how response scales are used, and iii) demographic differences in the profile of the respondents. Parallel surveys were conducted using CAWI and CAPI methodologies, and data were compared before weighting and following demographic weighting.

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