Open formats for data and metadata
Long-term archiving electronic data in a way that they can be accessed after some time (think rather in decades than years) is still an unsolved issue. It is increasingly being discussed by librarians and alike, and aspects include both, data formats as well as programs to read those formats and the hardware the actual data is stored upon.
Experience shows that there is only two general data formats that seem to be sufficiently long-term stable: plain text (including all kind of markup) and raw binary according to the IEEE 754 standard.
Regardless of the actual format used to store data and metadata, it is of paramount importance to fully document the format and to publish this specification open and free of charge. Otherwise, reproducibility cannot be guaranteed. Nevertheess, open formats are only a necessary requirement, but not sufficient for reproducible research.