In June 2018, NBC News and the Wall Street Journal reported the results of a poll about problems and priorities for the U. S. They used random-digit dialing to poll 900 registered U. S. voters nationwide. Those polled answered the question, "What do you think is the most important problem facing this country today?" From a list of 12 options, the largest majority of those polled (38%) said "health care." This is an observational study designed to answer a question about a population. Who is the population?