must come from the same population base as the cases; subtle differences in the way cases and controls are selected may lead to selection bias. The major point is that the controls have to reflect the population from which the cases arose. For general-population case-control studies, various methods are used to identify controls for study as discussed in Section 4.3.4.

4.1.4 Cohort Studies

In a cohort study, the investigator typically selects a group of exposed and a group of unexposed individuals and follows both groups over time to determine disease occurrence in relation to the exposure. In the radiation epidemiology field, when individual exposures or doses are available, cohort studies typically examine gradients of exposure rather than just un-exposed and exposed groups. The data necessary for assessing disease diagnosis can be obtained either directly by periodic examinations of individuals or by obtaining data from disease registrations, hospital records, and death certificates. For rare diseases or those that take a long time to become evident, such as cancer, the investigator needs to start with a large number of exposed and unexposed individuals and follow them for a long period of time. Study participants may be lost to follow up in a cohort study because they do not wish to take part in the study, because they cannot be located, or because they have died. Minimizing these losses is crucial because they reduce the number of participants being followed. Also, participants that are lost to follow-up may differ in characteristics from those that remain enrolled in the study. When reporting the study design, it is important to note the percentage of and any available demographic information on subjects that are lost.

A cohort study is considered to be a more scientifically rigorous study design compared to case-control, ecologic, or cluster studies. This is because cohort studies measure potential exposures before the disease has occurred and therefore can demonstrate that they may have caused the disease. Because cohort studies most often look forward to the future, they are also referred to as prospective studies. However, a cohort study can also be retrospective if both exposures and outcomes have already occurred and accurate historical data are available when the study begins. Studies on radiation effects are often jointly retrospective and prospective; exposures occurred mainly in the past and disease ascertainment includes both past and prospective follow-up.

4.2 STUDY DESIGNS CONSIDERED

Choosing from among different possible study designs to assess cancer risks in populations near nuclear facilities, or even deciding against making



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