Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Helmut Kromrey (2006). Empirische Sozialforschung. Modelle und Methoden der standardisierten Datenerhebung und Datenauswertung [Social Research Methods. Models and Methods of Standardized Data Collection and Data Analysis]