Published online by Cambridge University Press: 29 January 2018
Where preventable causes of a disorder are unknown, the epidemiologist will be concerned to search for an association between an environmental factor and some attribute of the disorder, commonly its incidence. In choosing what environmental factors to study, the epidemiologist will be attracted, firstly to those which on current hypotheses seem likely to have a causal role, and secondly to those for which the necessary data are easily collected. No doubt it is for the second of these reasons that studies have been made on the relation between schizophrenia and season of birth: the date of birth of a patient is rarely unknown, and the information, which may be accepted as being accurate in the great majority of instances, is routinely recorded in the case notes of most hospitals and clinics. All that the investigator has to do (it would seem) is to count the numbers of patients born in the different seasons of the year and compare these with the numbers to be expected from the season-of-birth distribution of the general population.
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