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In reality, most genetic studies rely on clinic‐based study populations because they provide the investigators with faster access to patients. There are numerous examples of genetic studies relying on this design. Generally, patients are selected for participation from existing patient populations, such as a specialty clinic (e.g. examining the phenotypic heterogeneity of age‐related macular degeneration in patients selected from ophthalmology clinics at two academic centers (De La Paz et al. 1997)). Less effort is required for the initial identification of the patients in this approach compared with the population‐based approach. However, the disadvantage of a clinic‐based study population is that it can be quite challenging to determine the larger underlying population from which a clinic population was drawn. Identifying appropriate control subjects for clinic‐based studies can be difficult for this reason, as discussed later in this chapter. Most large, academic medical centers attract patients not only from the immediate geographic region, but often from all over the world. A further complication of clinic‐based samples is that patients ascertained in this manner may exhibit a more severe form of the condition, simply because they have sought medical treatment or have been referred for expert diagnosis and care. For example, some individuals with myotonic dystrophy have cataracts as the only phenotypic manifestation (Meola 2013). Therefore, these individuals would not seek medical attention on a frequent basis. In contrast, other individuals with myotonic dystrophy are severely affected and seek medical treatment regularly. Consequently, ascertaining patients with myotonic muscular dystrophy from a neurology clinic would likely over‐sample individuals with the most severe form of the condition and omit ascertainment of the individuals who carry the less severe form of the condition (cataracts only). Thus, while clinic‐based populations provide fast access to desired patients, results obtained in such studies may not be applicable to the general population, or even to all individuals with the condition.