Researchers examined the records of a large number of cancer patients. Some patients received an invasive treatment, and the other did not. Patients who received the invasive treatment survived longer than patients who did not receive the invasive treatment. What can you conclude?

Let's Do It! 3.3 - Invasive versus Noninvasive

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Is this an observational study or an experiment?
Experiment
 Observational Study
Upon further investigation, it was found that only relatively healthy patients received the invasive treatment. What can you conclude now?