Methodologies for interventional myopia studies.

Myopia studies are notoriously difficult to carry out. Past studies on intervention in myopia progression have given conflicting results. Beside inaccurate and inadequate measurements, the most important cause for this is the very variable nature of myopia, which makes it difficult to achieve baseline comparability between the control and the study group. Although there were inclusion criteria in these studies, for age, sex, race, degree of myopia and stigmatism, the most important variate-- the rate of myopia progression-- was not included. Randomisation can achieve baseline comparability of the myopia progression rate, provided the sample sizes are large enough. Unfortunately, past studies have been limited to 100 to 200 children only. Studies on twins are more reliable than random groups because myopia progression rates are more likely to be the same in a pair of twins. Studies on the same subject, comparing the right eye and the left eye would be even better, but this method is practicable for some studies only (e.g., we cannot have a spectacle lens for one eye and a contact lens on the fellow eye). There is another method of doing an interventional study on myopia. Because myopia progression is linear in its early stage until the early teenage years, it is possible to observe what happens to the linear progression upon intervention. In this way, we avoid the problem of trying to compare "apples with apples" but use the "same apple" instead.