End-digit preference in blood pressure recordings of patients with ischaemic heart disease in primary care.

End-digit preference describes the disproportionate selection of specific end digits. The rounding of figures might lead to either an under- or over-recording of blood pressure (BP) and a lack of accuracy and reliability in treatment decisions. A total of 85 000 BP values taken from computerised general practice records of ischaemic heart disease patients in England between 2001 and 2003 were examined. Zero preference accounts for 64% of systolic and 59% of diastolic readings, compared with an expected frequency of 10% (P<0.000001). Even numbers are more frequently seen than odd numbers. In all, 64% of nonzero systolic recordings and 65% of diastolic recordings ended in even numbers, compared with expected proportions of 44% (P<0.0001). Among the nonzero even numbers, eight is the most frequently observed: 28% of systolic and 31% of diastolic recordings compared with an expected proportion of 25% (P<0.0001). Among the five nonzero odd numbers, five is the most frequently observed end digit, representing 59% systolic and 62% of diastolic compared with an expected level of 20% (P<0.00001). English general practice displays marked end-digit preference. This is strongly for the end-digit zero. However, there is more use of other end-digits, notably 8 and 5. This bias potentially carries important treatment consequences for this high-risk population.