The urge to introduce a 'missing' category is often strong. But a new category that references just one or only a few pages is not worth much. In these cases a common TopicPage should be enough, and given a more general category (if at all, see PleasePleaseDontCategorizeEveryPageOnWiki).

There should be a guideline for introducing categories.

I propose that if you want to introduce a category, then you should first pull together some (say 10) pages that already fall into it. And then you can add the category. This was the advice I was given when creating CategoryWikiProgress.

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