Last week, I gave a talk at Statistics Netherlands (slides here ) about panel attrition. Initial and nonresponse and dropout from panel surveys have always been a problem. A famous study by Groves and Peytcheva ( here ) showed that in cross-sectional studies, nonresponse rates and nonresponse bias are only weakly correlated. In panel surveys however, all the signs are there that dropout in a panel study is often related to change.
Instead of separating out mode effects from nonresponse and noncoverage effects through statistical modeling, it is perhaps better to design our mixed-mode surveys in such a way so that mode effects do not occur. The key principle in preventing the mode effects from occurring, is to make sure that questionnaires are cognitively equivalent to respondents. This means that no matter in which survey mode the respondents participate, they would give the same answer.