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Microphones and other equipment in a recording room. - 

This story was produced in collaboration with FiveThirtyEight.

The most listened-to podcasts come mostly from public radio: "Serial," "Invisibilia," "This American Life." But the ads? 

"Many of our advertisers are the same ones that you hear on Howard Stern," says Lex Friedman, who heads up ad sales at Midroll. "Literally, you hear Casper mattresses and Audible and Squarespace and ZipRecruiter." 

But this is something people like Friedman are trying to change. Ask about his dream advertisers, and he talks about categories like major movie studios, car companies, clothing companies — household name brands that you'll find on prime-time TV.

Why? 

"Here's the crass answer: they have deeper pockets, right?" says Friedman. "So, if you can get Coca-Cola or car companies or studios...they have big budgets."

Big budgets that let those advertisers buy in bulk — spending less per ad, but buying more ads overall. 

To see why that's desirable, you can look at the data set put together by FiveThirtyEight data reporter intern Hayley Munguia. She spent two entire days listening to the latest episode of every single podcast in the iTunes top 100—and writing down all the ads. There were 186.

But more than a third of the shows she listened to had no ads. And on the other two thirds, the median number of ads was just two. That's a relatively low "sell-through" (as the percentage of ads sold is called) for shows that have the six ad slots. Lex Friedman says  is common on Midroll's shows. 

“If a show gets 50 or 60 percent sell-through, we’re happy with that,” he says. “But we’re not satisfied with that." 

The FiveThirtyEight database reveals that the vast majority of today's podcast ads are, indeed the "Howard Stern" ads: host-read scripts for mid-sized companies selling online.

"For the most part I would say they all blended together, but yeah," says Munguia. "There were a few that stood out for being particularly ... terrible."

For instance: Bill Burr's pre-Valentine's Day endorsement of Sharri's Berrieschocolate-dipped berries you can order online. 

"Show her you thought of something unique and different this year, and get her the gift she is sure to love: Sharri's Berries," Burr says near the beginning of the ad on his Monday Morning Podcast. "Yeah, get her something unique. Get her something you and f**king four million other people are going to get."

When Munguia heard this ad, she wondered: has anyone from Sharri's Berries ever heard this ad?

The answer is yes. I played the ad over the phone to Sharri's Berries' acquisition marketing director, Nick Fairbairn. 

"I mean, does that fall within our brand standards? Probably not," he says. "Is it authentic to Bill Burr the comedian in a podcast space that's not nationally regulated for language? Yeah, it's right on brand for him. And gosh it was ROI positive, too."

ROI as in "Return On Investment" — as in, that ad actually sold a lot of berries. Fairbairn credits the ability of the endorsement (or endorsement-style) "native ads" that are typical on podcasts to blur the line between content and commerce.

"Keep it authentic, don't force it," says Fairbairn. "I think that's the key to doing this stuff."

And they know the ads work, because of a different part of the ad — something Bill Burr repeated at least three times: 

"Go to berries.com, click on the microphone and type Burr," he says.

These coupon codes are the norm for today's podcast advertisers. They give listeners an incentive to become customers, and they also give advertisers a handy way to track ads' effectiveness. (Especially useful, because data on podcast listening is less than solid.)

But the question is: by proving that podcast ads can move berries, do they also prove their value for companies that don't count coupons — like, say, Coca-Cola?

"By no means is it perfect for anybody," says Derek Lu, a senior strategist at the Media Kitchen, which buys ads for companies including Goldman Sachs. 

"It's hard to measure engagement; it's hard to measure and track the user journey," he says. And advertisers get relatively limited reach in exchange. "[Companies that advertise on podcasts are] there because they can reach a very niche audience where they couldn't otherwise have reached them," he says.

One possibility, according to Sherrill Mane, SVP of industry services for the Interactive Advertising Bureau, is that they will remain niche. 

"It's a craft, almost," she says. "And in the craft business sometimes if you get a high enough unit price you don't need to sell mass."

In other words, as long as the top podcasts can charge high rates—for host-read ads with coupon codes; selling berries, websites and stamps—they may be just fine without Coca-Cola.