Details: The privacy-oriented rival search engine DuckDuckGo had a group of volunteers search for the same terms — “gun control,” “immigration” and “vaccinations” — at the same time.

• Different users got different links. The company said in a report that some “people were shown a very unusual set of results relative to the other participants, offered some domains seen by no one else. If you were one of these people, you would have no way of knowing what you're missing.”
• The order of the results varied, with 76 people getting a total of 73 different sets of search results for “vaccinations” in both private and non-private browsing modes.

The company eliminated variations that could have occurred as a result of the search users’ location or, because the users made their queries at the same time, when they made the searches.

The big picture: Google has long touted personalized search results as a user benefit. But nearly a decade ago critics began noting that filtered search results can end up isolating users from information that doesn't match their world views in what Eli Pariser dubbed "filter bubbles."

What they're saying: "Google has claimed to have taken steps to reduce its filter bubble problem, but our latest research reveals a very different story," DuckDuckGo said in its report.

The other side: Google says it is working to improve the quality of its results, including in the boxes of featured content that appear at the top of search results.

• Google said in an email that instances of personalization in search are infrequent and most common when it comes to clarifying a vague search. It also said that differences in search results can occur thanks to many factors other than direct personalization for a user.