SOC 3673 SOCIAL EPIDEMIOLOGYESTIMATING RISK: IS THERE AN ASSOCIATION?Chapter 11The fundamental difference between a randomized trial and a cohort study is that, in a cohort study, subjects are not randomly assigned to be exposed or to remain non-exposed, because randomization to exposure to possibly toxic or carcinogenic agents clearly would be not acceptable.Regardless of which design is used, the objective is to determine whether there is an excess risk (incidence), or perhaps a reduced risk, of a certain disease in association with a certain exposure or characteristic.Risk can be defined as the probability of an event (such as developing a disease) occurring.ABSOLUTE RISKThe incidence of a disease in a population is terms the absolute risk.Absolute risk can indicate the magnitude of the risk in a group of people with a certain exposure, but because it does not take into consideration the risk of disease in non-exposed individuals, it does not indicate whether the exposure is associated with an increased risk of disease.Although absolute risk does not stipulate an explicit comparison, an implicit comparison is often made whenever we look at the incidence of a disease.HOW DO WE DETERMINE WHETHER A CERATIN DISEASE IS ASSOCIATED WITH A CERTAIN EXPOSURE?To determine whether such an association exists, we must determine, using data obtained in case-control and cohort studies, whether there is an excess risk of the disease in persons who have bee exposed to a certain agent.How can we determine whether an excess risk is associated with each of the food items? One approach is to calculate the ratioof the attack rate in those who ate each food to the attack rate in those who did not eat the food. An alternate approach is to subtract the risk in those who did not eat the food from the risk in those who did eat the food. The differencerepresents the excess risk in those who were exposed.To determine whether a certain exposure is associated with a certain disease, we must determine whether there is an excess risk of disease in exposed populations by comparing the risk of disease in exposed populations to the risk of disease in non-exposed populations. This can be done in two ways:1.The ratioof the risks (or of the incidence rates):Diseas erisk∈exposedDisease risk∈nonexposed2.The difference in the risks (or in the incidence rates):(Disaserisk∈exposed)−(Disease risk innonexposed)RELATIVE RISKThe Concept of Relative Risk
Both case-control and cohort studies are designed to determine whether there is an association between exposure to a factor and development of a disease.