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 andmeasurement. Relative to telephone-only, the offer of theWeb 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 financialmatters, 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.