At the recent SMX East conference, held in New York City, Google Webmaster Trends Analyst Gary Illyes announced that Google has started to roll-out the mobile-first index for very few sites.
Ilyes also mentioned that the aforementioned index is still in testing phase and hence the updates will be rolled out really slowly. Earlier for all the search rankings, Google had only desktop indexing there wasn’t any kind of mobile indexing.
To determine site ready to be added to the “mobile-first index”, Google has set up “classifiers”. These classifiers base the sites on how similar are the desktop and mobile versions of the website with regards to the content, links, schema, multimedia et al. If the classifiers are a 100% match, it is more likely that Google will consider the site in the initial phase of the rollout. Webmasters of websites with 80% match will be communicated of changes they may have to implement to get the website close to the 100% mark to be included in the rollout.
Gary Ilyes also mentioned that the reason for the slow and limited release is to carry out more tests. As of now, the tests seem to be going on well and over time more sites will be added to the index. Google has a blog in works to help webmasters with the new index and will also constantly communicate the changes along the way. As Ilyes mentioned, there is no foreseeable time as to when the mobile-index will be fully rolled out.