Data statements are used in publications to signpost where data directly supporting the publication can be found and under what conditions they can be accessed.
Many funders now require publications arising from publicly funded research to have such a statement. RCUK has such a policy and the EPSRC, specifically, have stated that from 1st May 2015 they expect a data statement to be included in published outputs.
University of Liverpool Research Data Management policy also states that published results should include a statement.
It is good practice to include a data statement in all publications, even if there is no new data. If all the data has been included in a supplementary file, your data statement should link to it.
If you are storing your data or the metadata about your data in a data repository (such as Liverpool’s Data Catalogue) then you should simply include the link to the appropriate webpage using the DOI generated on depositing the data. Thus, in the draft of your paper you can provide a statement without the DOI. Once your paper has been accepted, deposit the dataset in your preferred repository and obtain the DOI to insert in the final author accepted manuscript that you return to the publisher. Do not forget this is also the version that you should deposit into Liverpool Elements to comply with open access requirements.
If your data does not have a DOI, then the statement should include how the data can be downloaded, with an alternative web link or a departmental/group email address (you should not include a personal email address). Also state what conditions (if any) apply to the data, ie whether a general licence applies or whether a data sharing agreement must be entered into.
In some cases you may have to explain that access to the data is not available/appropriate.
If you did not collect the research data yourself but instead used existing data obtained from another source, this source should be credited.
The University of Bath has some good examples of data statements