Clinical or epidemiological studies in ophthalmology generally involve the examination of eyes for a variety of clinical signs or measurements. Unlike cardiology, hepatology, or many other clinical specialties, however, individuals have two eyes. In addition, unlike kidneys or lungs, both eyes are easily accessible for assessment and are frequently both assessed. In some instances a number of participants in a study may refuse complete assessment in both eyes, or the data from one eye may be unavailable for other reasons, resulting in data sets with information on one eye for some individuals and on two eyes for others.

If information is available on both eyes of the same individual, the findings in the left eye are generally likely to be more similar to those in the right eye of the same individual than to those in another eye from a different individual. This is because a multitude of factors, including environmental and genetic factors, act at the level of the individual and thus have an impact on the probability of the finding occurring in both eyes.

Clearly, however, the degree to which a given finding in the right eye predicts the probability of the same finding in the left eye varies considerably for different conditions. Some conditions characteristically only ever occur in one eye. This may be because the disease is rare. An example of this is choroidal melanoma which occurs in only one eye in 99% of cases.1 Other examples are corneal herpes simplex infection in the immunocompetent or severe ocular trauma (98% cases2).

At the other extreme are conditions such as blepharitis which almost always affect both eyes (proportion bilateral 95%3). The finding in the right eye almost perfectly predicts the finding in the left eye (fellow eye). The majority of ocular conditions lie …