Jacobs/Brown licensed the AC Nielsen ratings survey reports for every Star Trek episode aired on NBC. This information is published in the These Are the Voyages books, taken directly from the Nielsen reports. Other ratings data was taken from Arbitronand TvQ survey reports published in Variety and other trade magazines from the 1960s. The source is always cited.
The ratings services do not always agree. It sometimes seemed that even Nielsen didn't agree with itself, but this is only because Nielsen would routinely conduct several different surveys for any given week, including Trendex Overnight reports, Regional reports, 15-City reports, 30-City Reports, and National reports.
The data from any given report published in the These Are the Voyages books will differ from other reports from the same time period. When possible, data from more than one ratings report was included for various episodes.