how is it measured?
It is measured by the usage of digital video recording devices such as TiVos which indicates the time-shifted viewing.
Nielsen ratings

Changing systems of viewing have impacted Nielsen's methods of market research. In 2005, Nielsen began measuring the usage of digital video recording devices such as TiVos. Initial results indicated that time-shifted viewing (i.e., programs that are watched after the networks have aired them) will have a significant impact on television ratings. A year later, the networks were not factoring these new results into their ad rates because of the resistance of advertisers.

In July 2017, Nielsen announced that it would include select programs from subscription-based video on demand (vSVOD) services Hulu and YouTube TV in its Digital in TV Ratings system. Since about October 2017, Nielsen also began to track select programs from Netflix. Partnering distributors insert a "tag" into the program to be distributed on these services, which Nielsen then tracks through its meters system. Partnering distributors are able to determine if these ratings can be released publicly or not.
family guy - peter beats up chris's bully season 5 ep 11 UNANSWERABLE what is the tan aquatic with steve zissou? It is the eleventh episode of series "Family Guy". how was it received by the people? The episode was viewed in 8.53 million homes in its original airing, according to Nielsen ratings, Ahsan Haque of IGN gave rating 7.4/10. what is this ratings system? "Nielsen ratings" are the audience measurement systems operated by Nielsen Media Research that seek to determine the audience size and composition of television programming in the United States using a rating system. has it ever been criticised? Yes, there is some public critique regarding its accuracy and potential bias.
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Nielsen ratings [SEP] Measuring ratings
The original data collection methods used to generate Nielsen TV ratings included: Changing systems of viewing have impacted Nielsen's methods of market research. In 2005, Nielsen began measuring the usage of digital video recording devices such as TiVos. Initial results indicated that time-shifted viewing (i.e., programs that are watched after the networks have aired them) will have a significant impact on television ratings. A year later, the networks were not factoring these new results into their ad rates because of the resistance of advertisers. In July 2017, Nielsen announced that it would include select programs from subscription-based video on demand (vSVOD) services Hulu and YouTube TV in its Digital in TV Ratings system.