Fig. 1. Bias plot of (SpCO − COHb%) versus (SpCO + COHb%)/2. Typical single-subject data. SpCO is the Rad-57 measurement of carboxyhemoglobin; COHb% is the average of three CO-oximeters’ measurements of carboxyhemoglobin. Lines show values of bias ( solid ) ± precision ( long/short dash ).