In a study on the effect of exposure to insulation material on the risk of lung

Question

In a study on the effect of exposure to insulation material on the risk of lung cancer, the number of deaths from lung cancer occurring among 825 insulation plant workers employed between 1941 and 1944 was compared to the number of deaths from lung cancer occurring among 700 coworkers who did not work with insulation during the same time period. In a second study, the medical records of 50 insulation plant workers who died of lung cancer were compared to those of 100 coworkers who died free of lung cancer in order to assess if they worked with insulation.
Which one of the following statements is correct?

Both studies were controlled experiments.

Neither study was a controlled experiment.

Both studies were controlled experiments.

Explanation / Answer

Correct Answer: The first study was a controlled experiment, while the second was an observational study.

Explanation:

First study is certainly a controlled experiment because one group of 825 size is under the exposure of insulation material, whereas the other group of 700 size is not exposed to the insulation material. In other words, we are controlling the status of independent variable in both groups. Therefore it is controlled experiment.

However, in the second study, we are observing the data of all the insulation plant workers' death and identifying the ones who died of lung cancer vs those who died free of lung cancer. Here we do not have any explicit control over the independent variable's (exposure to insulation material) status. Hence second study is an observational study.

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