"This study was set up to examine whether personalizing advance letters by adding names and appropriate salutation, affects the survey cooperation of subgroups in the general population differently, in analogy to findings that subgroups react differently to advance letters. Differential reactions could be an explanation for the mixed findings in the literature on personalization of advance or cover letters. In a large scale study (N=39,518), information in communal registries was used to study (non)response patterns in subgroups, as a result of personalization. Advance letters of the Dutch CAPI Labor Force Survey were randomly assigned to addresses. In the non-personalized version (N=30,899), letters were addressed to 'the inhabitants of '. In the personalized version (N=8,619), the name or names of the household core were derived from municipal registries and used for addressing the letter. A re-interview addressed the issue whether the advance letter was read by more households when the household was addressed by name. By linking the sample to registries, it was possible to study response behavior of subgroups. The study focused on groups the literature indicates as differentially reacting to advance letters, i.e., different age, ethnic, gender, household composition and income groups, and groups with or without a listed telephone number. Hardly any difference in the overall level of cooperation was found if either a personalized or non-personalized letter was used. However, differential reactions were found for listed versus unlisted telephone owners, where only listed households reacted positively to personalization. In the other subgroups studied, no firm evidence of differential reactions were found. The paper discusses what these results signify for sample composition and the risk of bias." (author's abstract)