Ideas in this flowchart

• Cancer risk and protective factors
• It is usually impossible to know exactly why one person develops cancer and another does not.
• Risk factors may increase a person's chances of developing cancer.
• Risk factors include exposure to chemicals or other substances and certain behaviors.
• Risk factors also include uncontrollable factors such as age and family history.
• A family history of certain cancers may signal an inherited cancer syndrome.
• Protective factors are linked to a lower risk of cancer.
• Epidemiology studies initially identify most cancer risk and protective factors.
• Scientists compare large groups of people who develop cancer with those who do not.
• These studies may reveal differences in behaviors or exposures between the groups.
• Epidemiology studies alone cannot prove that a behavior or substance causes cancer.
• An observed association may result from chance or from another true risk factor.