Epidemiological studies aim to investigate factors while controlling for variables, though this can be difficult with human subjects whose lives vary greatly. Longitudinal studies follow the same group over long periods, like a US study tracking 100,000 children from birth to age 21 to examine environmental and genetic impacts on health. Evaluating scientific studies requires assessing valid methodology, precise measurements, reproducibility, potential biases, and disclosures of researchers and funders. Epidemiological research identifies risk factors for cardiovascular diseases, distinguishing between fixed factors like genes and modifiable factors impacted by individual choices.