Google is finally allowing Nielsen to put measurement tags on YouTube video ads, which will likely attract more advertisers to the site. Nielsen will be able to provide demographic information in partnership with Facebook, and marketers will be able to see how many people are seeing their ads, and how often. For two years, Google wouldn’t allow measurement tags on its ads, but the change of heart comes at a time when online ad sales are booming.

According to The Wall Street Journal, eMarketer reports that “online video advertising in the U.S. is expected to top $4.1 billion this year” — a 43 percent increase from just last year. TV spending will increase by 2.8 percent to more than $66 billion.

“We know our clients want meaningful measurement, which is why we’re investing in brand-friendly metrics,” Google said. “While we continue to build measurement options powered by Google, we’re also partnering with industry leaders, such as Nielsen and comScore, to offer objective, credentialed, third-party measurement options.”

Nielsen’s ratings will function similarly to TV audience ratings, which WSJ says many analysts believe is key for making the online ad sales boom a reality.

“If online premium video wants to steal TV dollars,” RBC Capital Markets analyst David Bank tells WSJ,  “they need to be able to transact in a currency that is similar to TV.”

After a short testing period, Nielsen says, the measurement tags will be accepted across all of Google’s properties by early 2014.