In a given dataset, a column of numbers can represent a categorical or numerical variable. While one can imagine some simple heuristics for guessing which (e.g., it's categorical if the number of unique values is less than some fixed threshold), I think it would be a fun exercise to train a classifier instead. Features to pass to the classifier might include the number of unique values and the total number of values.

The only difficulty is that I need a labeled data set of such features. It's probably too much to expect that such a meta-dataset already exists, so my question is: are there open data repositories that are sufficiently uniformly structured that I could pull the datasets and assemble the features in an automated way?

  • Can you provide examples of when numbers represent a category versus a variable? I guess I see a flaw with calculating percentages of distinct values as they could be categories or variables depending on the context of the column. It seems only a data dictionary could definitively explain what the column data is representing.
    – Sun
    Sep 5, 2014 at 6:18
  • You're exactly right, which is why I find the problem interesting. My idea is to use ML to find a heuristic smarter than a hard percentage threshold. Sep 9, 2014 at 3:27

2 Answers 2


Two things:

  1. Existing functionality: The two main open data portal software platforms I have worked with, Socrata and CKAN, have this functionality built in. Socrata is closed source while CKAN is open source.

  2. Check out data.gov, datahub.io, and others for a lot of open data to "train" from. Notably:

    • For Socrata API endpoints, such as https://open.whitehouse.gov/Government/2010-Report-to-Congress-on-White-House-Staff/rcp4-3y7g you can get column metadata at URLs such as https://open.whitehouse.gov/views/rcp4-3y7g/columns.json. Be careful though, this is no longer documented on dev.socrata.com from what I can tell and might be deprecated/unsupported
    • For CKAN, you need to find the resource ID of a datastore-backed resource, and you can create a URL such as and it will show you the metadata of the columns as well

Hope this helps!

  • Thanks for the detailed response. I haven't looked into (2) very carefully yet, but regarding (1): the column type inference in messytables is for determining the appropriate storage format (e.g., string, integer, float). My problem is a bit less trivial: is a given list of integers semantically categorial or numerical? In general, it is impossible to know, but there are probably some very good heuristics. Aug 30, 2014 at 20:23
  • Ah - I see. That is interesting and is probably more of a statistics question, IMHO, than an open data one. I would assume most numbers are numerical unless otherwise noted. Aug 30, 2014 at 20:25

I have found a useful resource for this project, the readMLData R package. It provides a uniform R interface to the UCI Machine Learning Repository. Crucially, it includes copious metadata for each dataset.

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