Use on questions that ask for analysis of one or more of the available data-collections both public and private. If your question is about a single observation use the tag [statistics] instead.
When you have a question that can't be satisfied with a simple statistic but would need multiple datasets, research and analysis the tag data-analysis should be used.
It can applied to both support and discussion type of questions. The former would be more to request for access to data-sets and/or the availability of such data-sets needed for your analysis.
The latter invites others to share/improve on ideas / proposals for data-analysis.
In all cases clarify what the reason is for the need to do a data-analysis and what benefit / evidence you expect to find and how that is helpful to the wider community.