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 and 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.

If it is only to satisfy your own curiosity see if ready available resource like Data Explorer, Google Big Query Stack Overflow and/or the Stack Exchange API