High blood pressure

Task

In a study of college freshmen, researchers found that students who watched TV for an hour or more on weeknights were significantly more likely to have high blood pressure, compared to those students who watched less than an hour of TV on weeknights. Does this mean that watching more TV raises one’s blood pressure? Explain your reasoning.

IM Commentary

The purpose of this task is to assess understanding of how study design dictates whether a conclusion of causation is warranted. This study was observational and not an experiment, which means that it is not possible to reach a cause-and-effect conclusion.

This task could be used as an assessment item, or it could be the basis of a small group or whole class discussion.

Solution

This does not mean that watching more TV raises blood pressure. Whether or not we can conclude that “watching more TV raises one’s blood pressure” depends on the design of the study. If the researchers had conducted a randomized experiment where some of the participants were randomly assigned to watch less than an hour of TV and others were assigned to watch more than hour of TV, and if we found a statistically significant difference in the average blood pressure of the two groups, we could conclude causation. If the study is simply an observational study (which is the case here), we can only conclude that there is an association between time spent watching TV and blood pressure.