Researchers divided a group of people on the basis of their smoking status (smokers and non-smokers) and followed them for 20 years to see if they developed lung cancer.This is an example of what type of epidemiological research design

 The research design described in the scenario is an example of a cohort study. In a cohort study, researchers follow a group of people (a cohort) over time to see who develops a specific outcome, such as a disease. The people are grouped based on their exposure to a certain risk factor or characteristics, in this case, smoking status. Cohort studies are prospective in nature because they follow participants into the future from the point of cohort selection and exposure assessment.

Cohort studies are observational in nature, meaning the researchers observe without intervening. They are particularly good at studying the effects of suspected risk factors that can't be manipulated for ethical or practical reasons. For example, it wouldn't be ethical to ask one group of people to start smoking to see if they get lung cancer. Therefore, researchers instead observe people who already smoke. Cohort studies like these can reveal associations between risk factors and disease development, but they can't definitively prove causation because other variables might influence the outcome.