Some beef "raised without antibiotics" tests positive for antibiotics in study

Naturally raised meat commands a huge financial premium. Consumers may not be getting what they pay for.

Excerpt from the article:

A new study in Science magazine identified antibiotics in some of the beef cattle in a USDAapproved no-antibiotics labeling program recognized as a gold standard for restaurants and grocery stores around the country. Page 4 of 6 The study tested some 699 cows at one slaughterhouse that processes "raised without antibiotics" cattle. Most cattle in the study tested negative for antibiotics. However, 10 percent of cattle came from lots where one of the cows sampled tested positive for antibiotics, the researchers found. Additionally, the study found an additional 5 percent of cattle came from lots with multiple positive antibiotic tests.

What measure was used to summarize the presence of antibiotics in beef cattle? A. Cumulative incidence B. Incidence rate C. Prevalence