Epidemiology is the study of the causes, distribution, and control of disease in populations.^{1}

An underlying assumption of epidemiology is that diseases and health outcomes are multifactorial, caused by many different variables or factors. These independent variables, which may include the health behaviour, toxic substance, or other event or material a person encounters or experiences are identified as the exposure. An outcome can be a disease or other health outcome or health behaviour. An epidemiologist investigates the relationship between the exposure and the outcome of interest and how the exposure changes the chance someone will experience the outcome.^{2}

The second underlying assumption in epidemiology is that health outcomes are not randomly distributed in a population. The multiple factors that cause a disease are measurable and identifiable.^{2} The information from epidemiological research therefore provides a factual basis for the development of rational public health policies for cancer control.

Key principles of epidemiology:^{2}

- Focus on the health of groups of people
- Used to describe where, when and to whom a health event occurs
- Used to quantify the amount of risk associated with a particular exposure or behaviour.

Common epidemiological terms^{1-3}

Incidence | The number or frequency of new health outcomes or health events over time |

Incidence rate | The rate at which new events occur in a population in a defined period divided by the population at risk (usually expressed as annual rates per 100000 population) |

Prevalence | The proportion of a population that actually has the disease (outcome) at a specific point in time. Calculated by dividing the number of existing cases by the total population at risk of the health outcome at a given point in time. |

Mortality rate | The number of a population that dies in a specified period, divided by the population at risk (usually expressed as annual rates per 100000 population) |

Survival rate | The proportion or survivors in a particular cohort, after a specified period of time (often measured over 5 years) |

Relative survival rate | The proportion of survivors in a particular cohort adjusted for other causes of death. |

Age-standardised incidence / mortality rates | Summary measures which allow comparison of populations with different age distributions, either different populations at the same time or the same population at different times. |

Risk factor | Any personal attribute, environmental exposure, or other feature of a person or his or her environment that increases the likelihood that he or she will experience a given health outcome. |

Protective factors | Any of the same types of variables (for risk factors) that reduce the chance a given outcome will occur. |