A few years ago a research study made headlines. Researchers were looking for a possible link between Vitamin D supplements and survival rates, so they had doctors around the United States ask their patients whether they took a vitamin D supplement. Five years later they followed up with the doctors to see which patients were still alive and which were not. Identify the population and sample of this study: The sample is the patients who took a vitamin D supplement. The population is patients who were surveyed by their doctors. The sample is the patients who were surveyed by their doctors. The population is patients who took a Vitamin D supplement. The population is American adults. The sample is American adults.