Tags around categorical data coding / representation

Question

Return back to the issue, June 30, 2017

I want to bring your attention once again to this tag question about categorical data encoding. It has not been resolved satisfactory so far, to me.

1) I claim that we do need one special tag for (various types) of categorical data encoding/representation in analyses (types are: dummy or one-hot coding, deviation or effect coding, Helmert, repeated, etc. etc.). One omnibus tag for them. Just imagine it: we have endemic tag [many-categories] but we don't have a tag for ubiquitous theme of how to represent categorical variables in this or that analysis or situation. We have currently only tag [categorical-data] which covers it but is much more broad and general.

So, I recommend to create the tag. Wording candidates: [contrast-variables] or [categ-data-encoding] or [categ-coding-scheme] or suggest your variant.

2) Dummy (aka one-hot) binary variables are the most well-known type of the categorical encoding / contrast variables. As currently, the tag [dummy-variables] is a slave synonym to [categorical-data]. Please recover it back as the tag on its own. Next thing: we yet keep currently separate tag [indicator-variables] however. This is strange because it is conceptually the same thing as dummy variables (just less known term).

So, either map [indicator-variables] to the restored [dummy-variables] as its junior synonym; or, if you prefer [indicator-variables], keep it, but then map [dummy-variables] to it (not to [categorical-data]).

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Original post Mar 6, 2017:

Please, consider these opportunities. Agree, disagree, your suggestions...

Moderators, please remove [dummy-variables] tag from being a synonym to [categorical-data]. These are definitely not synonyms. Categorical data can be represented as dummy variables, sure. That doesn't make the two concepts same at all.

[dummy-variables] should be kept a separate tag, and [indicator-variables] be made its junior synonym. In a recent question/comment I've expressed my tendency to see indicators as a synonym of dummy. "Dummy", in turn, is just the binary type of "contrast variables", not any contrast variables.

Therefore tag [contrast-variables] should be created. It is elementary variables to which categorical data (factors) are recoded, explicitly or implicitly, in ANOVA-like procedures. Contrast variables correspond, but not synonymic, to [contrasts] (comparing combinations of coefficients, we do have this tag already). Contrast variables include types "dummy", "deviation / effect", "Helmert", and so on.

If people agree and moderators recover [dummy-variables] I could then proceed and fix the tagging of questions according to the above thesaurus.

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So far I can see these three alternatives:

• define [dummy-variables] (or [dummy-set]) as binary indicator variables obtained out of a categorical variable. & create [contrast-variables] (or [contrast-set]) as more general - not necessarily binary - such variables (I vote for this variant)

• define [indicator-variables] (or [indicator-set]) to be what "dummy variables" was defined just above. & define [dummy-variables] (or [dummy-set]) to be what "contrast variables" was defined just above.

Both variants require [dummy-variables] to be restored as a proper tag.

• Third solution: don't restore/define anyhow old [dummy-variables] tag, however create tag [contrast-variables] which, if you like, we may give another name, say [categ-coding-scheme].

Answer 1

Update (26 Oct 2017): Master tag categorical-encoding has been created, and has the following three synonyms: [dummy-variables], [indicator-variables], and [one-hot].

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I claim that we do need one special tag for (various types) of categorical data encoding/representation in analyses (types are: dummy or one-hot coding, deviation or effect coding, Helmert, repeated, etc. etc.). One omnibus tag for them.

This suggestion sounds sensible to me. It got several upvotes (your question + some of the comments) and nobody has objected in the discussion above.

I think it would be good to choose a name that includes "categorical" in it. But [categ-data-encoding] and [categ-coding-scheme] sound too clumsy. Perhaps [categorical-coding]?

Apart from that, I am not a big fan of narrow tags, and I don't like subset tags. So I would be against maintaining further tags that are subsets of this main categorical-coding tag. Therefore:

• dummy-variables should be uncoupled from categorical-data (I agree with you here), but then made a synonym of our new categorical-coding tag. I don't think we should have it as a separate tag.

• indicator-variables (37, no excerpt) should be made synonym of the new categorical-coding tag too. [Note: many questions currently having this tag are about indicator function and are not at all related to encoding categorical variables for statistical analysis. Those have to be dealt with.]

  Update: indicator-function was created for questions about indicator function. Along the way, [indicator-variables] has been emptied, so if we want to make it a synonym of categorical-encoding, it has to be re-created first.

• We could create [contrast-variables] as another synonym of the same categorical-coding tag if we think it makes sense.

• The same goes for [one-hot]: we could create it as another possible synonym of the same tag, it might be useful.

This way we don't need to decide (see discussion in the comments above) on the terminological details such as whether "dummy variables" can have values other than 0 and 1. We'll have one omnibus tag for all these topics. It will be a subset of categorical-data, but perhaps you are right that the topic is important enough to warrant that.

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Just to mention it here: if we go with the above proposal, some manual retagging will be necessary to put this new "coding" tag on the most important threads about encoding categorical variables. As always, we don't need to put it everywhere; it's enough to put it on the most important threads. As far as I understand, @ttnphns volunteered to take care of it.

Answer 2

Do we need a special tag [dummy-variables]?

Since and thank to @amoeba's answer (which was well accepted, and I upvoted it too) we've acquired tag [categorical-encoding] - a separate tag marking topics on the ways to recode/represent categorical data in numerical data analysis.

One of the most well-known, ubiquitous way of "categorical encoding" is dummy variables (aka indicator variables, aka one-hot variables) - a set of binary variables - each one corresponding to one category - where 1 stands for "this category" and 0 stands for "not this category".

In their point 1 @amoeba thought we don't need a separate tag for dummy variables (I don't think we should have it as a separate tag) as soon as we have [categorical-encoding]. But I always believed we ought to have [dummy-variables] along with [categorical-encoding]. I don't think it is deadly critical, only that we'd rather have it than not, as it looks to me. It will be useful.

Reasons to have [dummy-variables]:

• Dummy-variables are most often used and well-known type of categorical coding. A person may even not be aware of different ways of categorical coding but he is likely to be familiar with "dummy" or "one-hot" terms.
• It is used far beyond the domain of representing of a predictor (factor) in ANOVA-like analyses; i.e. far beyond the theme of "contrasts". Almost every kind of data analysis which can manage categorical data will - internally or explicitly - encode it as the binary dummies. Thus, [dummy-variables] makes interest in threads discussing many algorithms, not just threads on categorical analysis.
• In "contrast" tasks (categorical predictor) we often consider k-1 set of dummy variables, not full k-variable set (k is the number of categories). It is an important (for parameter interpretation) issue which one is to drop in ANOVA context. But for analyses where we may use all k variables (clustering etc.) that question ("k or k-1 and which to drop?") doesn't belong to. Some users who think of [categorical-encoding] as contaminated with the question will be relieved to find tag [dummy-variables] which isn't necessarily contaminated and may use it calmly, say, in the context of cluster analysis.
• Sometimes the input is already a set of dummy variables and the task or the text contains just no word "categorical" anywhere. Therefore the issue that the set of numeric variables potentially represent one nominal variable is not apparent or important at all.

Note: if people decide that we create the tag, but they will oppress "dummy variables" (which term is OK for me), we could name that tag [indicator-variables] or [one-hot] instead.