Enable Statistics for a Subwiki
Steps
Storage of statistics is decided per wiki by the statistics field of that wiki's XWiki.XWikiPreferences object, and no administration screen creates that field. Adding it by hand is the only way to store statistics for one subwiki, and it takes two steps: the field has to exist in the class before it can be set on the object.
You need administration rights on the subwiki, and the statistics module has to be enabled for the instance, which it is unless xwiki.stats was set to 0. See Statistics Configuration.
- On the subwiki, open /bin/edit/XWiki/XWikiPreferences?editor=class.
- Type statistics in the "Add new property" field, leave the type on "Boolean", and click "Add". The property is added to the class straight away.

- Open /bin/edit/XWiki/XWikiPreferences?editor=object, expand "Objects of type XWiki.XWikiPreferences" (which is when its fields are loaded), set the statistics field at the end of the list to "Yes", and save.
- The field now reads "Yes", and the subwiki stores statistics from its next page view: the field is read on each request, so nothing has to be restarted, unlike a change in xwiki.cfg. Nothing reads those statistics: their only interface, the Statistics Application, has to be installed separately and is no longer maintained.

FAQ
How do I stop storing statistics for this wiki again?
Set the same field to "No". Setting it back to the empty value instead makes the wiki follow xwiki.stats.default again, which is 0 on a stock instance.
Does the main wiki need the same field?
Only if it should differ from the instance-wide default: a wiki without the field, main or not, follows xwiki.stats.default.