DuckDuckGo conducted a study in June on 2018, in which it carefully examined how Google processes search results for logged in, logged out, and anonymous users.
Although the study of DuckDuckGo may seem quite biased, given that even a search engine is a direct competitor to Google, their findings are interesting for anyone who wants to better examine Google's internal features.
According to the findings of the study, Google personalized results are displayed based on personal information previously collected about specific users (e.g., search, browsing and shopping history), automatically adding each of them to a so-called "filter bubble" .
Google uses this filter bubble to tailor particular search results that appear to a user who is searching for a particular term or phrase, so most of the study participants see results unique to them.
In addition, although Google claims that it worked to remove the "filter" function that showed the results that users would like, DuckDuckGo discovered, with the help of the 87 volunteers in the study, that users saw results uniquely in each, even when disconnected or in private browsing mode.
Users are trapped in Google's "bubble filter" even after logging out and browsing anonymously.
Google also showed some search results to some of the participants in the study, which other users did not see as if they were looking for them, although they were incognito or disconnected. This in itself means that if Google had eliminated the filter bubble bias, they would all have the same results, but this is not the case, and instead, by linking a user to the internet, the Google search engine uses the information it already knows about providing personalized results.
According to DuckDuckGo, using the private browsing feature or if you are disconnected from Google's account, do not expect the anonymity imagined by most people. In fact, you can not use Google search and avoid its bubble filter.
This is because fingerprinting techniques as well as IP addresses can be used to track people even when they think they are visiting the web anonymously. At this point it may be worthwhile to try one. Log in to a Google account, search with a random English term, and see 5 first search results. Disconnect, go to your browser in anonymous browsing and search with the same term. Finally, open Tor or a Proxy that has a proxy and do the same search again. Compare 5 first results for these 3 searches.
All the DuckDuckGo data collected during the study, as well as the instructions and search results achieved, along with the code used to analyze the information, are free and available on its website and are ready to be examined by anyone interested in verifying their findings.