The first step toward determining if something is helpful or harmful to human health is to design a study. Basically, if you want to assess the link between a risk factor and a particular disease, you have two possibilities: you can start from the risk factor and try to connect it to the disease, or start from the disease and try to work back to the risk factor that caused it.
Let us first discuss starting from the disease and trying to identify the risk factor. We will continue with our example of tobacco. If you think that tobacco causes lung cancer, you can go to the hospital and ask patients with lung cancer if they smoke or used to smoke. This information by itself is not very useful, because we need to compare it to the same information obtained from healthy people without lung cancer. To do this, we need to find healthy people who have other characteristics (like age, gender, profession, hobbies, etc.) that are as similar as possible to the group of lung cancer patients.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2020.00015