Introduction
Epidemiology is the medical science interested in what causes a disease and how it spreads among people. This topic covers risk, risk factors, exposure, odds and confounding factors; the concept of causality and how events can be associated; and the Bradford Hill criteria for judging whether an association is causal.
Risk and risk factors
- Risk: the probability that an event will occur. In epidemiology, it is the probability that a person exposed to certain factors (risk factors) will subsequently develop a particular disease.
- Risk factors (RFs): characteristics associated with an increased risk of developing a disease or other health-related event.
- Exposure: contact with RFs before developing a disease; it can take place at a single point in time or over a period of time.
- Odds: the ratio of the probability of one outcome to the probability of another outcome.
Risk is measured from 0 to 1: 0 means it will not happen, 1 means it will happen. Almost nothing is 0 or 1; values lie in between (the one certainty is that every person who is born will die). Risk is the chance (ratio) that the event will happen.
Confounding factors
A confounding factor (CF, a "third variable") is associated at the same time with a risk factor and with the disease, but is not itself a risk factor for the disease. The main confounders are sex and age.
- Example: people who drink and smoke have a higher risk of lung cancer. Smoking is the risk factor for lung cancer; alcohol is the confounding factor.
- Example: being a man is not a risk factor for prostate cancer; it is a confounding factor.
- Example: in a flu epidemic, newborns and the elderly can die while adults survive thanks to a strong immune system. Age is associated with the disease outcome but is not the risk factor.