Conversion factor calculation and validation for field method 11. (A) Visual inspection of the data used for the conversion factor calculation showed that all values generated by field method 11 were higher than the reference method, indicating a consistent bias between the methods. (C) Bias plot of the same data. If there were no difference in the estimated mean bias, the dotted line would be at 0. The values generated by field method 11 were, on average, 4.3-fold higher than the values generated by the reference method. The conversion factor of 0.23 was the antilog of the mean bias. The conversion factor was validated by subsequent sample exchange. Visual inspection of the converted validation data (B) and a plot of the bias (D) indicated the estimated mean bias was close to 0. The field method generated values that were on average 1.2-fold lower than the reference method, indicating a considerable improvement in data alignment. It was estimated that 95% of values were within the range of 4.9-fold lower to 4.5-fold higher than the reference method. Significantly, 59% of all samples were within plus or minus 2-fold of the reference value. Before conversion only 14% were within 2-fold. The conversion factor calculation required the X variable to be the field method. However, for the validation process the reference laboratory data were the X variable to determine the relative difference.