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The hypothesis that habitual physical activity protects against coronary (ischaemic) heart disease is derived from statistical comparisons which show that incidence, prevalence, and mortality rates vary inversely with the average levels of exercise involved in different occupations (Morris, Heady, Raffle, Roberts, and Parks, 1953). Similar findings were subsequently made in a variety of studies of occupa- tional groups in several countries (Brown, Davidson, McKeown, and Whitfield, 1957; Morris and Craw- ford,
Yasin et al. (1967) studied this question.