What's the adjectival form of "data"/"datum"?

In Latin, datum is already an adjective of sorts, in so far as it is a form of the verb do (to give) and means 'that which is given'. If you had to make an adjective of it in English, you would have to use given, as in 'a given fact'.

I would probably use data-generating if I had to.

How about statistical?

I realize it's not the same root word, but my dictionary¹ shows a credible thread:

statistical (adjective): of or relating to the use of statistics

statistics (plural noun): the practice or science of collecting and analyzing numerical data in large quantities, esp. for the purpose of inferring proportions in a whole from those in a representative sample.

statistic (noun): a fact or piece of data from a study of a large quantity of numerical data

So long as the data is numerical or quantitative, at least, that word seems to work (although it might be more of a stretch if applied to, say, demographical data.)

¹_{(Definitions from NOAD; emphasis added.)}