A claim, mind you, that is very bold. Getting millions of voters to vote a particular way is the sort of thing that political parties spend a lot of time trying to figure out. Epstein is claiming that more than 2 percent of all 2016 voters were influenced to vote for Clinton by Google. The scale is massive.
So why does he make this claim? He appears to have combined two bits of research he’s conducted: a 2015 look at how search engine results can influence political opinion and a collection of search results from users before the 2016 contest. Over the last 25 days of the campaign, a summary of the latter research suggests, “we found that search results were, on average, biased to favor Hillary Clinton on all of those days.” Given that his earlier research found results could influence views of candidates, we get the top-line assumption.
What does “biased to favor Hillary Clinton” mean? We don’t know. The summary doesn’t explain what that looks like.