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Letter: Ocular disease in South West Africa.

Sir,-The report by Jeffrey Freedman on ocular disease in South West Africa (Brit. J. Ophthal. (I973) 57, 68i) is superior to most reports of this type in that the population studied is defined, data are reported by age and sex, and an effort was made to examine a representative group. Two comments seem appropriate. It is difficult or impossible to know who may be representative with respect to ocular pathology (or any other characteristic of interest), and the very best way to avoid bias is to select at random. Since a preliminary house-to-house census was included as part of the study, it is most unfortunate that some procedure was not utilized to select patients for examination. When the investigator chooses subjects he thinks are representative, he may succeed or he may fail by a wide margin. There is no way to judge. Random samples are not only unbiased, but the probability that the sample findings deviate by a designated amount from the underlying population is readily calculable. The common mix-up between prevalence (cases existing at the same time of the examination) and incidence (new cases occurring during a specified time period, such as a year, without regard to whether they have been cured or are otherwise not present at the end of the period) is maintained in this paper. The summary paragraph refers to "a survey of the prevalence. . . ". Elsewhere the paper refers to "the first account of the incidence . . .". In fact the paper provided only prevalence data.