American market-research engineer and business executive, best known for the “Nielsen ratings,” which offer a national rating of television viewing.

Nielsen's parents were both accountants, and he acquired mathematical interests at an early age. In 1918 he graduated from the University of Wisconsin and then worked briefly as an engineer. In 1923, with the financial backing of his fraternity brothers, he founded A.C. Nielsen Co., which eventually became the largest market-research concern in the world. In the early years the company had difficulties, almost going bankrupt twice, but it finally established a business foothold by analyzing retail food and drug sales, which thereafter remained the company's largest and most profitable operation. A.C. Nielsen Co. entered the radio program rating field in 1942 but became best known to the general public for its television rating service, which from 1950 gauged the popularity of television shows.

The rating system was based on a sampling of more than 1,000 television homes scattered around the United States. Each member of the sample had a small box, called an Audimeter, attached to the set, which recorded when the set was on and what channel was tuned in. These data were relayed to a computer centre, which also collected data from viewing diaries kept by a smaller sampling of households. Based on such information, the Nielsen raters projected a total audience for each program, as well as the age and sex of the viewers. A Nielsen rating of 20 denotes that 20 percent of all American households equipped with a television tuned in to a program. A “share,” by contrast, denotes what percentage of all the viewers watching television at a particular time tuned in to a particular program; a 30 share means that 30 percent of the viewing audience watched that program.