ABSTRACT
This chapter introduces the basic epidemiological concept of calculating and comparing risks and discusses some confusing definitions. The situation becomes simpler if a group of people who do not ordinarily eat together share a common meal, and some of them become ill afterwards. The chapter shows how one could analyse such a situation by calculating risks and relative risks. In a similar fashion, factors influencing attack rate can be studied by looking at the number of secondary cases in different groups around a primary case. All guests were sent a list of the food that had been served and asked to indicate what they had eaten. The chapter looks at the risk of being ill in those who did not eat the items on the list. The risk associated with some potentially harmful factor is defined as the proportion who becomes ill out of all those exposed to it.
