von der Lippe, Elena, Patrick Schmich, and Cornelia Lange. “Advance Letters As a Way of Reducing Non-Response in a National Health Telephone Survey: Differences Between Listed and Unlisted Numbers”. Survey Research Methods 5, no. 3 (December 22, 2011): 103–116. Accessed April 24, 2024. https://ojs.ub.uni-konstanz.de/srm/article/view/4657.