A study suggested that a combination of high blood pressure and recreational drug use could be lethal. The study followed 8000 men aged 55 and over for two decades and found that men with high blood pressure who participated in recreational drug use even occasionally had double the risk of dying from heart disease when compare to men with normal blood pressure who abstained from recreational drug use. Is this an observational or experimental study? The study is an observational study because the researchers only observe values of the subjects and do not apply any treatments. The study is an experiment because the researchers assign subjects to certain experimental groups and observe the outcomes. The study is an experiment because the researchers can not control what happens to the group after two decades. The study is an observational study because the researchers cannot control what happens to the groups after two decades.

To determine whether the study is observational or experimental, let's analyze the provided options and the nature of the study: 1. **Observational Study**: In this type of study, researchers observe subjects without manipulating any variables or assigning treatments. They collect data based on what naturally occurs. 2. **Experimental Study**: In this type of study, researchers actively assign subjects to different groups and apply treatments or interventions to observe the outcomes. Now, let's evaluate the statements: - **The study is an observational study because the researchers only observe values of the subjects and do not apply any treatments.** This statement is true. The researchers are observing the subjects over time without manipulating any variables. - **The study is an experiment because the researchers assign subjects to certain experimental groups and observe the outcomes.** This statement is false. There is no assignment of subjects to groups; the researchers are merely observing. - **The study is an experiment because the researchers cannot control what happens to the group after two decades.** This statement is misleading. The inability to control outcomes does not make it an experiment. - **The study is an observational study because the researchers cannot control what happens to the groups after two decades.** This statement is true. The lack of control over the subjects' experiences supports the classification as an observational study. Based on this analysis, the correct conclusion is: **The study is an observational study because the researchers only observe values of the subjects and do not apply any treatments.**