Observational precision in general practice data: a technique for analysis and audit.

Terminal digit preference is a well-known source of error in blood pressure measurements. This was demonstrated in general practice computer databases by conducting a routine search and constructing a frequency distribution of the 20 992 blood pressure readings using mathematical software for a personal computer. The charts produced have a practical application in providing an audit tool whereby nurses and doctors can be made aware of this source of error, providing an opportunity for improving measurement technique and developing more refined and realistic protocols for blood pressure management in general practice. The same principles may be applied to other measurements made in the GP consultation such as peak expiratory flow rate.