Improving between-wave mailings on longitudinal surveys: A randomised experiment on the UK Millennium Cohort Study

Abstract

Typically, longitudinal survey managers send ‘keep in touch’ mailings to sample members between waves to help minimise non-response through failure to locate individuals at future waves. This paper reports the results from a randomised experiment to improve the effectiveness of the between-wave mailing on the Millennium Cohort Study (MCS), a large-scale birth cohort study in the UK. Our experimental intervention, which involved revising the content of the covering letters used in the 2010 mailing, aimed to increase the proportion of sample members responding to the mailing, particularly among those with higher attrition rates i.e. lower educated sample members and minority ethnic groups. The re-design involved making the letter easier to read, due to a concern that poor literacy or English may be a barrier to returning the form for some groups, and changing the style and signatory, motivated by the psychological concepts of reciprocity, linking and helping tendencies. Our main finding, that the design of the covering letter has a minimal impact on the effectiveness of these mailings, will help guide further research in this area and help to inform practice on longitudinal surveys. The results also make an important contribution to the existing evidence on the content of advance letters more generally and thereby have broader applicability for survey research and practice.