When scientist want to gather information on the link between diet and cancer, they often rely on people self-reporting what they have been eating over a set period of time. This could mean finding a group of people and following them over time, checking in with them and asking what they’ve been eating, or it could mean retrospectively asking them to describe what types of food they have been eating over the last few years. The scientists will also collect information on anyone who has been diagnosed with any type of cancer during the period of time covered by the study.
Usually, they will also collect information on other factors associated with cancer, such as family history, bodyweight, and occupation, so that they can account for the influence of these factors when looking for links between diet and cancer.