“We’re not arrogant about the landscape and about the needs of marketers and media companies and agencies to have better, more comprehensive data,” said Steve Hasker, Nielsen’s global president chief operating officer. “We’re much more focused on meeting those needs and executing against our road map than we are looking over our shoulder.”

For a long time, the only real recourse television and ad executives had was to grumble. They still relied on and paid for Nielsen measurement because it was their main option.

Now, new rivals seeking to challenge Nielsen’s dominance have emerged. The latest came on Monday, when comScore and Rentrak announced they were united after the closing of their merger. ComScore, founded in 1999, has expertise in measuring the use of digital media, while Rentrak uses data from set-top boxes to measure what people watch on television. Combined, the companies are seeking to measure audiences across screens — mobile, desktop, television and more.

Whether any of the new players will dethrone Nielsen is unclear. Some industry analysts like Mr. Nollen and other executives said that because of its legacy, Nielsen remained best positioned to provide a single source of ratings data across TV and digital. Others question whether ratings will even be relevant in a not-too-distant digital future, when ads are bought and sold based on specific data about viewers, such as location, occupation, salary and purchases, rather than broad audience metrics.

“The road is littered with the roadkill of companies that have tried to compete with Nielsen over the years,” said Alan Wurtzel, president of research at Comcast’s NBCUniversal. “Now, some of these new entities might wind up being able to do things in a way that Nielsen can’t. If they have figured out a better mousetrap, they could give the industry something to seriously think about.”

In the last year, Nielsen has released 69 new products and technology innovations — most notably introducing a measurement called total audience metric to track TV viewing across video on demand, mobile and streaming.