What are research data?

Short Answer

Research Data comprises all data that is either used or produced in research processes.

Detailed Answer

In order to understand the term research data a general definition of the term data is necessary: Data is a set of measured values of qualitative or quantitative variables regardless whether it is recorded on paper or in digital format.
Although metadata is data about data and the related (data transformation) processes, it should be considered as essential as the primary data of a study, because metadata could be the primary data of someone else's research (e.g. in a meta analysis).

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