ABSTRACT
One group consists of people who possess some special attribute thought to be a possible risk factor for a disease of interest, whilst the other group does not. Both groups are followed over time and the incidence of disease compared between the groups. Hence, in a study of the hazards of working in the coal industry, a group of coal miners and a second group of employees in other heavy industries might be selected at baseline. Both groups would then be monitored for (say) 10 years, after which time the incidence of (say) bronchitis is compared between the groups. The nonfactor group (other heavy industries in the example) is included to act as a control group for purposes of comparison — that is, to enable the excess morbidity associated with the risk factor (coal mining) to be found. Figure 5.1 gives a schematic representation of this type of cohort study.
