SAUSALITO, Calif. — Marketers continue to struggle with the problem of mobile advertising attribution: that is, defining which piece of promotional (or social) content viewed on a mobile device ultimately led to a purchase.

In the early days of the web, attribution was straightforward, said American Eagle Outfitters chief digital officer Joe Megibow, speaking at the Mobile Summit conference in Sausalito, California on Monday. Marketers could easily track which ads were creating the clickthroughs that led to purchases.

But these days, consumers typically use more than one desktop computer, and often more than two mobile devices. They can see ads on any of these devices. They might also be influenced to buy by someone they trust on a social network or by a cool product they saw on Pinterest.

It’s very difficult for marketers to identify users across different devices, and to credit some piece of content as the thing that moved the consumer to buy a product. Without that information, the marketer doesn’t know what platforms to invest in.

Are mobile experiences leading to purchases? Megibow referenced an IBM study that states that, while 50 percent of consumers use mobile devices for shopping, far more actual purchases are happening on the desktop. Purchases on the desktop outpace mobile purchases by three to one, the study said.

“If mobile is growing like crazy but it is converting only a third as well as on the desktop, you have a serious problem,” Megibow said. “The data is saying ‘don’t spend any more marketing dollars on mobile,’ but that’s obviously wrong.”

Criteo chief revenue officer Mollie Spilman had some of her own data on mobile conversions. Criteo believes that 27 percent of sales are now taking place on smartphones.

And she believes that number will keep going up. “More sales are going to happen on smartphones than tablets or desktops,” she predicted.

Spilman pointed out that part of the issue is cultural. In Asia far more purchases take place on mobile devices than in the U.S. But Spilman believes this is changing, and that mobile purchases will become far more common for U.S. consumers as they become more comfortable with the security of the purchases, and as mobile payments technology becomes easier to use.

While all of that might make American Eagle Outfitters feel better about continuing to invest in mobile marketing, it still doesn’t fix Megibow’s attribution problem.

Megibow said his company is dealing with it by changing the way it looks at attribution in general. “Some of it has been just relaxing our philosophy,” he said. Megibow believes that purchases on the desktop are going up, but that content and interactions on mobile platforms are contributing to that trend.

So American Eagle won’t stop investing in mobile marketing anytime soon. Megibow sees both the mobile device and the desktop computer as important components in the sales process.

And digital sales and engagement continue to go up. Half of American Eagle’s customer touches each day happen through digital channels. So something is working.