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Somebody has just created tag by posting this question: Is every correlation matrix positive definite? Previously this tag did not exist, but of course there is well over a hundred older threads that could in principle be meaningfully tagged with it. I wonder if it is a good tag to have around, and if so, whether it should be slowly "propagated backwards".

Note that we do have and , but only used to have until now.

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    $\begingroup$ If you search for questions only, then it's 400 candidate threads, not 800. Still a lot of course. I have recently decided to create a [factor-rotation] tag and propagated it backwards on ~10-15 threads (out of many more possible candidates) that I thought are most important. With [correlation-matrix] this would probably be several dozens of "important" threads to mark, but certainly well below 100. $\endgroup$
    – amoeba
    Nov 22 '15 at 14:34
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    $\begingroup$ I've often wondered why we had 'covariance matrix' but not 'correlation matrix', but the task of putting it where it was needed seemed daunting. While I wouldn't suggest trying to put it everywhere it might be used, the strategy of identifying a few dozen questions it might be most important to have it on is interesting. We might then just put it where we see it's needed after that. Perhaps it's less than ideal (I don't think we can get ideal at this point) but I think it would help people find questions they're looking for. $\endgroup$
    – Glen_b
    Nov 22 '15 at 22:21
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    $\begingroup$ This resonates with my feelings well, @Glen_b. I started with suggesting a tag wiki excerpt. Unless somebody protests here in the next day or two, I might look for some threads to add this tag to. $\endgroup$
    – amoeba
    Nov 22 '15 at 23:56
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    $\begingroup$ I think it is a reasonable tag to have. Propagating it backwards can be beneficial but I guess it is something it can happen slowly and naturally as older questions resurface. $\endgroup$
    – usεr11852
    Nov 24 '15 at 23:38
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After the brief discussion in the comments above, I wrote a tag wiki excerpt for and tagged a dozen of old threads with it, where it seemed the most appropriate. So it seems the tag is here to stay.

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