To mix or not to mix data collection modes in surveys.

Traditionally in social surveys and official statistics data were collected either by an interviewer visiting a respondent or through a self-administered mail questionnaire. In the second half of the twentieth century this picture changed rapidly. Telephone surveys became increasingly popular, reaching their peak in the 1990s. Advances in computer technology in the last thirty years have made computer-assisted survey methods possible, including methods for Internet and web surveys. This variety of data collection methods led to methodological questions, such as, which method to choose? which is best? Recently in survey practice multiple modes of data collection or mixed-modes have become more and more popular. In this article I will outline the advantages and disadvantages of mixed-mode survey designs, starting with an overview of common forms of mixed-mode design and discussing reasons for using more than one mode in a survey. This overview will end with a discussion of practical issues and an agenda for future research.