it has been suggested that drivers have a higher risk of road accidents on 'critical' days in any of their supposed physical, emotional or intellectual biorhythm cycles; the risk increases when 'critical' days in any two cycles coincide and is greatest when three 'critical' days coincide. an investigation of the theory was carried out at the transport and road research laboratory using data from an insurance company on a large sample of drivers involved in accidents. the sample consisted of 112560 drivers of whom 96283 were men (group m) and 16277 women (group f); 39586 of the men (group mb) and 6876 of the women (group fb) were considered blameworthy, 51254 men (group mnb) and 8504 women (group fnb) were considered not to blame; the rest were classified as 'blame not known'. statistical tests were made for groups m, mb, mnb, f, fb and fnb, of various combinations of possible 'critical' days in each separate cycle and in each two cycles taken together. convincing evidence was not found to support the biorhythm theory. accidents occurring when three 'critical' days coincide have not been fully studied but one example indicates that these occur very rarely.(a)

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