Be aware that when people speak of television ratings they are most often referring to those done by Nielsen Media Research which rates the audience size and composition of television audiences in over 40 different countries.
Know the lingo. The word "rating" refers to the percentage of viewers who watch any given show or time period based on the total number of people watching TV. A television program's "share" is based on how many homes have television sets and the number of those sets tuned in to any given show. A program or network's "reach" is the number of unique viewers or households that tune into any given channel or program. Nielsen also reports demographic information such as the age, sex, income level and education of viewers.
Understand how the data is obtained. Nielson utilizes different measurement tools in different markets. Primarily, they rely on diaries completed by individual viewers, though in some areas viewers may have people meters that record what they watch and when.
Do the math. Nielsen uses scientific sampling to compile their ratings information. For example, a random selection of viewers with different demographics will be used as a representation of the market as a whole.
Recognize that television networks and advertisers use ratings information to determine the best placement of a program or commercial as well as whether or not the show should continue.