The mobile-first index has started to slowly roll out, at least for a “few sites,” Google Webmaster Trends Analyst Gary Illyes told an audience last night at the SMX East conference in New York City.

It is unclear how many sites have already switched over to the mobile-first indexing process, but, when I asked Illyes to clarify what he meant by “a few sites,” he said a few relative to the Google index. So that could be quite a large number of websites.

He said there is no reason to panic about the rollout because Google is still testing it and will be rolling it out incredibly slowly. He added that there is no foreseeable time when the mobile-first index will be fully implemented.

The admission came as a surprise as we did not expect the rollout to begin until next year, but it seems positive test results have encouraged Google to move forward.

In selecting sites to be switched over, Google has set up “classifiers” to define how ready a site is for the mobile-first index. Classifiers determine how equal or comparable the desktop site is to the mobile site when it comes to content, links, schema, multimedia, etc.

If the content, links, schema and so on all match at a 100 percent level, Google is more likely to take that site to the mobile-first indexing stage. If they are at an 80 percent level, Google might wait and communicate to the webmaster that there are specific changes that need to be made to the mobile site to get it closer to being 100 percent comparable.

In any event, Gary Illyes said the purpose of rolling it out to a limited number of sites is to test it more. The tests seem to be going very well, and it will gradually roll out to more sites over time. He said the rollout will go incredibly slowly, and Google will communicate the process to webmasters along the way.

Google also has a blog post in the works that will be published at some point helping webmasters and SEOs understand the process. The company won’t provide a timeline or ETA for the rollout or the blog post, but Illyes confirms they are on the way.