Bland-Altman parcels were also used to investigate a possible link between the differences between the measurements and the actual value (i.e. proportional distortion). The existence of proportional distortion indicates that the methods do not uniformly correspond to the range of measures (i.e., the limits of compliance depend on the actual measure). To formally assess this relationship, the difference between methods should be reduced to the average of the two methods. If a relationship between differences and actual value has been identified (i.e. a significant slope of the regression line), 95% regression-based agreements should be indicated. [4] Bland-Altman plots are widely used to assess the agreement between two instruments or two measurement techniques. Bland-Altman plots identify systematic differences between measures (i.e. fixed pre-stress) or potential outliers. The average difference is the estimated distortion, and the SD of the differences measures random fluctuations around this average.
If the average value of the difference based on a 1-sample-t test deviates significantly from 0, this means the presence of a solid distortion. If there is a consistent distortion, it can be adjusted by subtracting the average difference from the new method. It is customary to calculate compliance limits of 95% for each comparison (average difference ± 1.96 standard deviation of the difference), which tells us how much the measurements were more likely in two methods for most people. If the differences in the average± 1.96 SD are not clinically important, the two methods can be interchangeable. The 95% agreement limits can be unreliable estimates of population parameters, especially for small sampling sizes, so it is important to calculate confidence intervals for 95% compliance limits when comparing methods or evaluating repeatability. This can be done by the approximate Bland and Altman method [3] or by more precise methods. [6] The Bland and Altman (BA) methods (Reference Bland and Altman 1) have been used regularly to assess the relative adequacy between two dietary methods. The reason is usually that the reference method – or the gold standard – is considered more accurate, but it must also analyze a considerable burden for participants and resources.