Epidemiological research methods. Part IV. Case-control studies.

Cases* Controlst Total instead, available lists (cancer registers, admissions to hospital, notifications or registered deaths) are used, but then one cannot be sure that all cases have been assembled-for example a list of hospital admissions for ischaemic heart disease or for traumatic brain injuries would exclude deaths before' admission as well as mild episodes either not treated or treated by general practitioners. To ensure that cases on the list are 'true cases', clear, objective diagnostic criteria for inclusion need to be used. lo In Schrek er al.'s study' the 154 cases were defined as all patients diagnosed as having cancer of the larynx, pharynx or lung who presented to the hospital between 1942 and 1944. In 50% of the lung cancer patients and 70% of the laryngeal and pharyn-geal cancer patients a histological diagnosis was made. By not using histological diagnoses for all cases the possible relationship between cigarene smoke and only a particular histological type could have been missed. When possible, it is desirable to use a case definition that maximises the number of true positives. However, the more rigorous the diagnostic criteria, the less likely is complete case ascertainment since access to specialised diagnostic centres is limited to specific subgroups; hence there is a trade-off between diagnostic accuracy and complete case ascertainment. Misclassification of case status (because of low diagnostic accuracy) will usually decrease one's chance of fmding an association between an exposure and a disease. It is preferable to include only new (incident) caseslO,11 so that uniform diagnostic criteria can be applied. The completeness of case ascertainment affects the potential of cases to be representative for all exposures. 2 For example, a hospital study investigating the relationship between train accidents and head injuries will miss the majority of accident victims who die immediately, and underrepresent trains as a risk factor for head injuries.-Lung, pharyngeal and laryngeal cancers. tAil other tumours. Randomised controlled trials (RCTs), although regarded as the best method for assessing the efficacy of an intervention, I have several shoncomings 2 ,3 and may be impossible to conduct, for example in the case of harmful risk factors or aetiological factors such as smoking4 or schistosomes. 5 Analytical studies such as case-eontrol studies or non-RCT follow-up studies 6 provide alternative approaches for assessing drug or vaccine 7 efficacy, health service 8 interventions, as well as the role of risk factors for disease. 4 ,5 Using these approaches …