and are two different tags on CV, with the first one reserved for kernel smoothing and density estimating, and the second one for kernel trick in maching learning (kernel SVM, kernel PCA, etc.). I noticed that a lot of people confuse these tags, mostly by using instead of more specific .

This is bad, because e.g. questions about kernel PCA are currently divided (approximately evenly) between these two tags. I see two possible solutions:

  1. is removed as a separate tag and becomes a synonym of .

  2. The questions confusingly tagged should be re-tagged.

Personally, I think that the tags are separated quite well and would therefore prefer the second option. Moreover, I was going to go ahead and re-tag all questions about kernel PCA to (and possibly some other questions about SVMs, if I come across any). This would, however, bump all these questions to the front page, and if I re-tag them in one sitting, then there will be lots of questions on the same topic edited by me appearing on the front page practically one after another.

Would this be okay?

[Of course, I can also make some other improving edits and maybe even answer some of these questions along the way, to make this less of a book-keeping activity. But still.]


There are currently 68 questions tagged with and and 7 questions tagged with and . Whereas I was originally referring only to questions on kernel PCA, the problem seems to be much more severe in the SVM topic. For comparison, there are only 16 questions tagged with and .

Update 2

Turns out, this issue was discussed two years ago: What does the kernel tag refer to? where @mbq and @whuber argued that is unnecessary. I think I am happy to accept this point of view, but that would mean that should be merged with .

Update 3

I just realized that there exists tag (used 45 times), which could most likely cover almost all uses of tag that are not referring to kernel trick...

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  • $\begingroup$ In respect of Update2: conclusions might change in the space of a couple of years; it's probably worth revisiting the question of how to deal with this. I imagine that the arguments there still largely hold up, but as the community grows, sometimes the need for particular tags can change. $\endgroup$ – Glen_b Sep 16 '14 at 23:17
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    $\begingroup$ Would it make sense to rename kernel to, say, kernel-smoothing? $\endgroup$ – djs Sep 17 '14 at 3:06

I would be in favor of option 2. The existence of 46 (as of 9/16/2014) tagged threads suggests that the tag has some organic utility for organizing the topics within CV.

However, I would make two suggestions:

  1. Don't re-tag all the candidate threads at once, so as not to spam the main page. It is best, IMO, to do a handful at a time. As a further extension of that idea, it is best to do the re-tagging during slower traffic periods.
  2. I suspect part of the problem is that has no tag wiki excerpt, and the excerpt for is too sparse. The excerpt for could be clarified a little, and something like 'DO NOT USE this tag for [kernel-trick]' could be appended to the end. Likewise, a suitable excerpt for should be created.
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  • $\begingroup$ Thank you. I was going to see what other people would say, but as your answer remains the only one, I went ahead and provided wiki excerpts for both tags with the "do not use" bits. I am also going to migrate all kernel PCA questions to kernel-trick in the next days as there are only few of them, but I don't think I am up to migrating all 60+ kernel SVM questions (as I never answer SVM questions)... $\endgroup$ – amoeba Sep 18 '14 at 10:02
  • $\begingroup$ Turns out there is also a kernel-density-estimate tag used 45 times, which makes the situation even more confusing (as the kernel tag basically decomposes in two). Any thoughts about this one? $\endgroup$ – amoeba Sep 19 '14 at 10:24

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