Sir,

The authors of the letter made several points regarding the validity of our results. We agree that a correlation coefficient plot does not necessarily exclude systematic bias or disagreement between measurements obtained by the two methods being evaluated. This is the reason for quantifying agreement using the Bland–Altman graphical method.1 In our paper we elected to use analogue measurements on the horizontal axis as this was regarded as the gold standard.2 Re-plotting the graph using an average of analogue and digital on the horizontal axis did not make any difference to the limits of agreement.

We found that the limits of agreement for distance up to 5 mm were clinically acceptable, but we do accept that there appears to be a linear relationship between amount of disagreement and magnitude of distance measured. We are grateful to the authors for pointing this out, and would suspect that the most likely source of this bias might be the actual screen size (number of pixels) setting on the computer monitor. This would explain the similar gradient seen in group 1 and group 2 plots. We will conduct further studies to evaluate the influence of screen size setting as a confounding factor.