A revised scheme is described for evaluating automatic instruments used in clinical chemistry. Procedures are outlined for the assessment of mechanical and electrical features, and measurement of the accuracy and precision of individual units. Methods are given for the measurement of analytical precision, carryover, cross-contamination, accuracy, and linearity. The safety of equipment and methods of assessing costs are discussed, and the importance of subjective features is noted. The general principles of the evaluation scheme should be applicable to other types of equipment.

SUMMARY Twenty five biochemical and haematological measurements were determined on nonfasting blood and serum samples collected between 9 am and 7 pm from a representative group of 7685 British middle-aged men. Most measurements showed significant diurnal variations, but only for bilirubin, phosphate, and triglyceride did time of day account for more than 5% of the between subject variance. Serum bilirubin concentrations showed a pronounced downward trend in the afternoon, the mean value after 6 pm being 30% lower than the mean value in the morning. Mean serum triglyceride and phosphate concentrations increased steadily through the day. Mean concentrations of potassium, haemoglobin, and haematocrit and red cell count were higher in the morning, while urea and creatinine concentrations and white cell count had higher means in the afternoon. Glucose showed a pattern consistent with short term response to meals. The British Regional Heart Study includes 7735 men aged between 40 and 59 years, who had been randomly selected from the age and sex registers of representative general practices in 24 towns in England, Wales, and Scotland. The criteria for selecting the towns, the general practice, and the subjects, as well as the methods of data collection, have been reported elsewhere.6From each practice's age-sex register some 420 men, aged between 40 and 59 years, stratified into five year age groups of equal size, were selected at random. The list of names was reviewed by the doctors in the practice, who were asked to exclude those whom they thought would not be able to participate because of severe mental or physical disability. The remaining subjects were invited to take part in the study in a letter signed by the general practitioner. Seventy eight per cent of those invited attended for examination by a team of three research nurses over a period of two and a half years.

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