The article “How Dangerous Is a Day in the Hospital?” (Medical Care [ 2011]: 1068– 1075) describes a study to deter-mine if the risk of an infection is related to the length of a hospital stay. The researchers looked at a large number of hospitalized patients and compared the proportion who got an infection for two groups of patients— those who were hospitalized overnight and those who were hospitalized for more than one night. Indicate whether the study is an obser-vational study or an experiment. Give a brief explanation for your choice.