Hi,

I am new to this forum and hope someone can help me with this issue:

I want to compare two methods with a Bland Altman plot. The difference between measurement 1 and measurement 2 are non-normally distributed data, therefore, I want to log-transform the data. I get the bias, SD and LOA on the log-transformed data, and can make the BA-plot.
My problem is when I am transforming back the LOA to original scale, which I want to insert on the original BA-plot, for easier interpretation. How do I calculate the slopes of the LOA and how do I know the intersection with the y-axis?

I also want to make 95 % CI of the mean difference of this data, how can I do this on log transformed data when I want the 95 % CI to be on the original scale? (for comparison to other normally-distributed data). Or should I just calculate 95 % CI of the mean difference on non-normally distributed data?

Thanks!!

Morten