A taxonomy of barriers experienced by users. Barriers may occur at many different steps along the path of re-use. For instance, an author may decide not to archive data, due to the perceived burden. If the author does not archive data, then it is difficult for users to discover that the data exist. Once the user discovers that the data exist by reading a publication, the only way to obtain the data is to write to the author, a process that is known to be subject to delays and refusals. Even if the data are placed in an archive, it may be difficult for users to discover (e.g., journal web sites typically do not offer any kind of content searching for supplementary data) or to access (e.g., users may be required to pay for access). Finally, it is not unusual for archived data to contain errors and ambiguities that make it difficult to apply in scientific research.