ESPN and Nielsen will now track TV and digital viewing in one number

The Disney-owned sports network is the first company to sign on to start selling ads using Nielsen Total Audience data

The reporting will take three weeks to complete initially

ESPN said it plans to start reporting a single audience number when it airs games – one that encompasses people who watch on TV, on digital devices, and in bars.

Here's the catch: It's going to take three weeks to get these numbers from Nielsen.

ESPN is the first media company to sign on to report on and transact using Nielsen Total Audience numbers. That means instead of reporting one set of numbers for TV and then another set of less compatible numbers for streaming, ESPN and Nielsen will be able to provide a single number representing the total number of fans who tune in to watch a live game.

The new initiative kicks off next week, when ESPN will report numbers for "Monday Night Football."

The total audience data won't arrive overnight, explained ESPN Global Sales & Marketing president Ed Erhardt in a conference call on Friday. On Tuesday, ESPN and Nielsen will release an initial audience number for the Monday Night game between the Dallas Cowboys and Arizona Cardinals that will represent live cable TV viewing and viewing through web-based TV services like Sling TV or PlayStation Vue.

A week later, ESPN and Nielsen will report another number that includes people who watched the game on apps via tablets and mobile devices.

Finally, on October 18, ESPN and Nielsen will be able to layer in an out-of-home viewing number – including people who watch at bars, restaurants, and hotels – and thus will be able to say how many people watched the game in total.

Going forward, that new comprehensive number will be used to make guarantees to advertisers, which is a significant step forward in the way that digital media is bought and sold. In the past, ESPN often ran different or fewer ads on digital platforms. But now all the ads will run seamlessly, regardless of device.

"We feel very good that we can look the industry in the eye," said Erhardt. "We'll have a third party measuring ESPN on every screen."