The impact of using the Web in a mixed-mode follow-up of a longitudinal birth cohort study: Evidence from the National Child Development Study

A sequential mixed-mode data collection, online-to-telephone, was introduced into the National Child Development Study for the first time at the study's age 55 sweep in 2013. The study included a small experiment, whereby a randomised subset of study members was allocated to a single mode, telephone-only interview, in order to test for the presence of mode effects on participation and measurement. Relative to telephone-only, the offer of the Web increased overall participation rates by 5.0 percentage points (82.8% vs. 77.8%; 95% confidence interval for difference: 2.7% to 7.3%). Differences attributable to mode of interview were detected in levels of item non-response and response values for a limited number of questions. Most notably, response by Web (relative to telephone) was found to have increased the likelihood of non-response to questions relating to pay and other financial matters, and increased the likelihood of ‘less desirable’ responses. For example, response by Web resulted in the reporting of more units of alcohol consumed, and more negative responses to subjective questions such as self-rated health, self-rated financial status and well-being. As there was evidence of mode effects, there is the potential for biases in some analyses, unless appropriate techniques are utilised to correct for these