As you now know, analytic epidemiological studies can be observational or experimental in design. Observational studies simply involve the investigator observing the populations or individuals. They don't interfere or manipulate the exposure in any way. One such study design is a case control study. Now I will discuss some key features of case control study designs. A case control study involves comparing individuals with a particular condition or disease, known as the cases, to a group of individuals with the same general characteristics but without the condition or disease of interest known as controls. Information on past exposure to possible risk factors is obtained for both the cases and the controls, and the frequency and intensity of exposure in the cases is then compared with that in the controls. The starting point of most case control studies is the identification of cases, however prior to selecting cases clear eligibility criteria should be defined, based on the objectives of your study. This is referred to as the case definition, for example, you may only be concerned with a population within a certain age bracket or a specific gender. Cases can be sourced from a variety of places such as hospitals, clinics or the community setting, however, you must be aware of capturing all representative cases, for example not just those that are more advanced that make it to surgery. These cases should be representative of everyone with the disease under investigation. Usually it is not too difficult to obtain a suitable source of cases but selecting controls tends to be more problematic. When you're selecting your controls, you should consider both the source of the controls and assessment of exposure. Control should come from the same study population as the cases and should be representative of the population at risk and exposures within controls should be measurable with similar accuracy in the cases. Assessing exposure in cases and controls has to be carefully considered. Self reported recall of usual behavior may not be comparable in cases and controls, for example if you have a chronic illness such as cancer, you may be more motivated to find out why you got the disease and thus think about your past differently and more likely to report it differently compared to if you did not have cancer or were a control participant. This is called recall bias. Another important factor is how many cases and controls are required. The number of cases that can be studied is often limited by the rarity of the disease being studied. If this is the case statistical confidence can be increased by having more than one control per case. As a result studies often allocate 2 or more controls per case. The advantages of case control studies are: they good for studying rare diseases because you can identify all of the existing cases that have already accrued over many years; they are relatively inexpensive to conduct; they can be quick to obtain data because you can assess exposure and outcome all at the same time. However they have disadvantages, and these include: there can be bias associated with exposure assessment, that is, the presence of disease may affect how an individual reports past exposure; there's often difficulty in selecting a good control group, and they are limited to assessing just one chosen outcome. They also can't tell you any information about the temporal relationship between exposure andthe disease. Hopefully you now understand the key features of designing a case control study. Next you will learn about how to measure associations in case control studies.