ABSTRACT
This chapter discusses the concept of a case control study and odds ratios. The entity used to measure how common an exposure is called odds, which is defined as number exposed divided by number not exposed. The analysis begins by calculating the odds connected to each exposure for the cases and for the controls separately. The most annoying thing about odds is that they have a less intuitive meaning than the concept of risk. The fact that sampling from big populations introduces an uncertainty in the numbers calculated is one of the main reasons why statistical methods are so prominent in epidemiology. Case control investigations for infectious diseases have, for example, been used to study risk factors for hepatitis C markers, and case control methods have also been suggested for the study of vaccine efficacy. When trying to think about causation it is often helpful to make the distinction between necessary and sufficient causes.
