Particular survey modes put respondents into different frames of mind, referred to as a mental "script".
For example, social desirability bias tends to be highest for telephone surveys and lowest for web surveys:[2][3] Therefore, as the data collected on sensitive topics (such as sexual behavior or illicit activities) will change depending on the administration mode, researchers should be cautious of combining data or comparing results from different modes.
By contrast, in telephone interviews, respondents are often limited by their working memory and are unlikely to understand a long question with multiple sub-clauses.
However, this is not always the case, and appropriate adaptation of questions to a new mode can yield comparable data [citation needed].
Unless experiments are specifically designed to investigate differential coverage, mode effects will be confounded by coverage [4] , and significant differences between modes/experimental conditions could have several explanations: This problem is exacerbated when in 'live' administration of a survey, multiple modes are used.
In this case, mode effects are difficult to quantify as randomly allocating respondents to a condition does not reflect their preference.
Such an experiment lacks external validity and results would not directly generalise to situations offering respondents a choice.