In an interesting study published last year, Smith et al evaluated airway volume, soft-palate area, and soft-tissue thickness before and after rapid maxillary expansion. They explored the reliability of measurements made on 3-dimensional images and used the intraclass correlation coefficient (ICC). In this letter, the rationale of applying the Bland-Altman plot and related suggestions to its use instead of the ICC are given.

Validity and reliability of data according to the type of variables (categorical or continuous) can be measured with various indexes. The ICC is one of the most frequently used indexes of reliability for continuous variables. It measures an absolute or true agreement between 2 or more methods or observers. This possibility in case of measuring absolute agreement rather than only consistency and correlation makes it a more appropriate tool for the assessment of reliability. Moreover, the ICC considers not only consistency, but also some systematic patterns between methods. However, its indication to measure both reliability and validity of continuous measurements compared with the Bland-Altman plot is questionable.

Accordingly, I would like to recommend that Smith et al should apply the Bland-Altman plot on their data to provide more information regarding the extreme and outlier values, the magnitude of disagreement, and its pattern between 2 or more methods.