View source for 17. Epidemiological indicators II: definition and measures of relative risk and odds ratio

* Risk is a proportion; odds is a ratio
* Risk
** = the probability that an event will occur
** Risk = number of times an event occurred / total number of possible occurrences
** Estimated based on incidence
** If there are 10 people with cancer, and 6 die, the risk of dying is 6/10 = 60%
* Odds
** = the chance that an event will occur
** Odds = number of times an event occurred / number of times the event didn’t occur
** If there are 10 people with cancer, and 6 die, the odds of dying is 6/4
* Indicators of association between the disease and its risk factors
** Relative risk
** Odds ratio
** Proportional mortality ratio
** Standardized ratio
* Relative risk (RR)
** The proportion of risk for a particular disease in the risk factor-exposed and non-risk factor-exposed population
** Basically how much higher the risk of disease is in those who are exposed to a certain risk factor
*** If RR > 1 there is a positive association between factor and the disease, so it is a risk factor
*** If RR = 1 there is no association between factor and the disease
*** If RR < 1 there is a negative association between factor and the disease, so it is a protective factor
** RR indicates the strength of the association
** Example from John Snow’s water pump:
*** People who drank water from the pump had 58,4% risk of cholera, while those who didn’t had only 6,7%
*** The relative risk is 0,584/0,067 = 8,72. The risk of cholera was almost 9 times higher in those who drank from the pump
** Used in cohort studies
* Odds ratio (OR)
** The ratio of the odds of having the disease in the population exposed to the risk factor and the odds of having the disease in the non-exposed population
*** If OR > 1 the disease is more likely in exposed than non-exposed population
*** If OR = 1 the disease is equally likely in exposed and non-exposed population
*** If OR < 1 the disease is less likely in exposed than non-exposed population
** Used in case-control studies to measure the strength of association between risk factor and disease
*** In case-control studies, relative risk can’t be calculated. However, for rare diseases, the OR is almost equal to the RR and is therefore used to estimate it

[[Category:Public health 2 - General epidemiology and demography]]