An important challenge of social science survey research is to define characteristics (phenomena) of individuals or groups of individuals that are not directly observable and to make them measurable. While it is easy to record observable, factual or objective characteristics (e.g. number of children, income) relatively error-free, this is much more complicated with characteristics that are not directly observable. These include personality traits, cultural capital or political trust. Measuring instruments for such characteristics must be constructed and documented with particular care to ensure the quality of their measurement (more precisely: objectivity, validity and reliability). This article explains how to construct and document such measuring instruments and explains why it is important for research and practice to make documentation of measuring instruments as freely accessible as possible (“open access”).