# Do we need a “dummy variable trap” tag?

To be honest, I was not even aware of the term "dummy variable trap" until a few days ago, but it seems like the term is popping up more and more frequently at CV (note chonological ordering).

Do we need a tag for this? Maybe look for a canonical answer (e.g., this one, though it goes into many other aspects of collinearity besides the "trap")?

• This is mostly a question of how categorical variables should be encoded. We have a relevant tag already: categorical-encoding. What might be helpful is to make a [dummy-variable-trap] tag & make it a synonym of [ce]. – gung - Reinstate Monica Jul 3 '19 at 11:24
• I think it's econometricians' or economists' jargon but (1) I would be happy to get better informed (2) that's not a criticism, just a description. But as you say, it's not everyone's standard jargon, which gives me a little bias against making it a tag. – Nick Cox Jul 3 '19 at 17:57
• FWIW I put a bounty on the linked answer by @Silverfish. – amoeba says Reinstate Monica Jul 3 '19 at 18:31
• @NickCox: I think that if a term is specific to a field, that would bias me towards creating a tag, because then we could explain exactly this in the tag wiki. (I am an incurable optimist: I still believe some people actually read those tag wikis. Someday I'm going to propose a new closure reason: "Question is completely answered in the tag wiki of one of its tags.") – Stephan Kolassa Jul 3 '19 at 20:29
• My bias is unreasonably that I can't get excited about tags whereas conversely I quite enjoy copy-editing which clearly is of little or no appeal to many. Seriously, I am all in favour of people who like working hard on tags doing what they like and do well. I've come across a case in another part of SE in which a poster got flak most unfairly for a post on Meta giving some generic advice relevant to particular software, say FOO, from people knowing nothing about FOO. So, they promptly put the same material in a tag wiki where it is safe as non-FOO people would never dream of looking there. – Nick Cox Jul 3 '19 at 21:11
• If you think it's an answer, I can convert my comment above. – gung - Reinstate Monica Jul 4 '19 at 12:21
• @gung: I would very much appreciate it. – Stephan Kolassa Jul 4 '19 at 13:43
• I would really appreciate making the dummy variable trap as a synonym. It would help people not familiar with the topic. The answer to this question for example feels a little off until you realize that their talking about categorical encoding and multicollinearity. (When I first read it, I thought he was talking about something singularities in outlier detection. This is very silly, I now realize, but still.) stats.stackexchange.com/questions/415841/… – mlane Jul 17 '19 at 1:14
• Update: I have created a dummy-variable-trap tag and started to retag a few questions at a time to avoid bump-flooding the landing page. I am a bit doubtful whether synonymizing it to categorical-encoding makes sense, so I'll leave that decision to people who actually have upvotes on the tag. – Stephan Kolassa Jul 17 '19 at 9:10

This is mostly a question of how categorical variables should be encoded. (The 'dummy variable trap' is simply that there is perfect multicollinearity if you use a separate dummy code for every level of a categorical predictor when there is an intercept in the model; see my answer at: Qualitative variable coding in regression leads to “singularities”.) As such, we have a relevant tag already: , although it's also related to . What might be helpful is to make a [dummy-variable-trap] tag & make it a synonym of [categorical-encoding].