1.
von der Lippe E, Schmich P, Lange C. Advance letters as a way of reducing non-response in a National Health Telephone Survey: Differences between listed and unlisted numbers. SRM [Internet]. 2011 Dec. 22 [cited 2024 Apr. 19];5(3):103-16. Available from: https://ojs.ub.uni-konstanz.de/srm/article/view/4657