That’s based on a Quartz review of 68,000 Facebook ads from Feb. 12 to 25. We didn’t read all those ads, but instead trained an artificial intelligence algorithm to tell us what all those ads were about based on the text within the advertisements.

Here are examples of each candidate’s ads promoting their top topic:

an ad from Pete Buttigieg that says "From combating gun violence to ensuring there is no such thing as an uninsured American, together we can bring about the change this country needs and turn the page."
An ad from Pete Buttigieg on gun control and healthcare
a Bernie Sanders ad that says "As President, Bernie Sanders will continue his lifelong fight to guarantee health care as a right to every man, woman and child, raise wages, lower prescription drug costs, and create an economy that works for all, and not just the 1 percent."
An ad from Bernie Sanders about healthcare and the economy.
an ad from Elizabeth Warren that says "President Obama called Elizabeth “one of the country’s fiercest advocates for the middle class.” After the big banks crashed our economy in 2008, Elizabeth had the political courage, original thinking, and persistence to call for a new agency to fight for consumers. That’s why President Obama picked her to create the Consumer Financial Protection Bureau — to protect consumers from financial tricks and traps often hidden in mortgages, credit cards, and other financial products. That agency has now returned $12 billion to people who were cheated by big banks and other financial firms. Elizabeth understands what’s broken, she knows how to fix it, and she’ll fight to get it done. Click here to learn more about Elizabeth."
An ad from Elizabeth Warren about the economy.
an ad from Tom Steyer that says "A recent Oxfam report says global inequality has reached disastrous levels. It calls on governments around the world to increase taxes for the wealthy and put people over profits. My tax plan will make sure the wealthy pay their fair share, which means we can give a 10% tax break to 95% of Americans. Join our campaign to create an economy that works for the people."
An ad from Tom Steyer about the economy.
An ad from Joe Biden that says "Vote Biden. Beat Trump."
An ad from Joe Biden about beating Trump
an ad from Amy Klobuchar that says "If you think women aren’t electable, you haven’t been paying attention. Women can beat Donald Trump. Take it from Amy -- she’s never lost an election, and with your help, she’ll win again in November. Donate now."
An ad from Amy Klobuchar about beating Trump

Tom Steyer has spent so much money in South Carolina that if you examine just the estimated number of ad impressions in the state, they drown out the stats from other candidates.

Quartz is using AI to help us cover online political influence, and in this case used an AI-based classifier to “read” the text and guess which topics are discussed in each ad. Unlike algorithms based on pre-defined keywords, this one is more powerful because it can learn new keywords. For instance, even if we didn’t tell the model that the term “GI bill” relates to both education and veterans, it will detect that the phrase occurs in ads with words like “veterans” and “college tuition” and learn what “GI bill” means. That can also lead to hiccups: for some reason, our model thinks that the word “Nevada” means an ad is about veterans.

Also, the text inside Klobuchar’s and Biden’s ads don’t dive particularly deeply into individual issues—though both run ads that discuss issues within videos, which our AI can’t read.

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