# Need for and name of [acf-pcf] tag

@gung recently created the tag which now has 9 questions: 2 old threads tagged by @gung and 7 new threads (plus 1 migrated to SO). I have two questions about the tag:

1. Do we need it?
2. If we need it, can we at least replace pcf with pacf in the tag name?

Edit: 2. Has been done already, only 1. remains relevant.

Regarding 1., I think we were doing more or less OK without it. Most of the questions containing ACF and/or PACF I have encountered on CV would ask for

If we were to get rid of the tag, it could be

• made a synonym of or
• just replaced by the above-mentioned tags in the few threads tagged with but not the ones above, and deleted in other.

Regarding 2., PACF is the standard acronym of partial autocorrelation function in time series econometrics, while I cannot remember seeing PCF before (though I would not be surprised it exists). On Cross Validated, PACF can be encoutered 670 times while PCF only 17 times. I think having PCF in place of PACF in a tag name may be confusing and may spread the unwarranted use of the nonstandard acronym (so that instead of searching for PACF one will now have to search for both PACF and PCF to capture all the relevant posts).

• I don't have an opinion here, but FWIW when gung created this tag, he seems to have put it on two threads: stats.stackexchange.com/questions/106038 and stats.stackexchange.com/questions/108983. The other 8 threads with this tag appeared later. Most of them do not have [autocorrelation] tag. Would your suggestion then be to make [acf-pcaf] a synonym of [autocorrelation]? Jun 19, 2018 at 12:49
• @amoeba, this could be logical. Either we keep it (then I would urge to change the name to acf-pacf) or make it a synonym, and then the intuitive synonym for me is indeed autocorrelation. Jun 19, 2018 at 13:04
• Seeing that many people started using this tag after it was created, I think having it around is a good idea. The question remains whether to make it a synonym or not (on which I have no opinion). Jun 19, 2018 at 13:57
• 7 questions over 2 months (besides the two old threads @gung tagged himself when creating the tag) – not too intense, but not dead either. Jun 19, 2018 at 14:02
• OK, the tag's name has been changed to acf-pacf. Jun 19, 2018 at 17:33
• I seem to have a vague memory that part of the point was to make it a synonym, but I'm not really sure. Couldn't someone have a question about the plots w/o it being a question about autocorrelation? What if someone wanted to ask, 'how to identify the MA parameter from this ACF plot?', should the system retag that (silently) to [autocorrelation]? Jun 19, 2018 at 17:43
• @gung, good example. Perhaps it shouldn't. Jun 19, 2018 at 17:53
• ACF and PACF are typically used in identifying ARIMA orders in a Box-Jenkins type approach. Looking (only) at the titles of the acf-pacf questions, it seems like most of these could be retagged box-jenkins. To be honest, I have little preference either way, since Box-Jenkins is really not the modern way to go about ARIMA model selection - nowadays, one uses information criteria. Jun 21, 2018 at 0:43

I am open to making it a synonym, but I'm also not convinced it should be a synonym of [autocorrelation] per se. I don't know enough about the topic to be confident of making any definitive statements, but it seems to me that someone might want to ask a question specifically about the plots themselves and that allowing the organization of such material by having the tag can be useful. In particular, they aren't really synonyms, in the literal sense. That doesn't mean that they can't be made into tag synonyms, but does suggest we should think it through first. For example, I could imagine someone asking, 'how to identify the MA parameter from this ACF plot?'. I wouldn't want such a question to be silently retagged to .