Agreement between Brüel&Kjær 1312 (eCOBK) and PiCO + Smokerlyzer (eCOPiCO) (Bland Altman plot). The measures taken into account were 102 (3 measurements for each of 34 subjects). The regression line for the mean agreement (Y = −0.3336 X −9.192) is reported. The coefficient of determination R2 was ≈ 0.7214 (p = 0.000). The standard deviation of the residuals is 2.2716; there is no significant relationship between the standard deviation of the differences and the average of the two methods (p = 0.165). To consider the repeated measures approach and the non linear relationship between difference and mean, the Limits of Agreement (LOA) was evaluated modelling the variability in the Standard Deviation of the mean difference directly as a function of the level of the measurement.