Choroidal Metrics Statistical Analysis

MATLAB (MATLAB 2020a; The MathWorks, Inc.) was used for statistical analysis. Normality of the data was tested with the Lilliefors test. For the analysis of topographical distribution, measurements from all nine scans were averaged per patient and are presented as a median and interquartile range. The quartile-based coefficient of variation³⁹ (CV) was calculated (equation 5) where Q₂₅ and Q₇₅ denote the 25th and 75th quantiles of the distribution.
$CV=\left(\frac{Q_{75}-Q_{25}}{Q_{75}+Q_{25}}\right)\times 100.$    Correlations of the choroidal parameters with AEL were performed using Spearman rank correlation.⁴⁰ (link) The coefficient of repeatability (CR) from nonnormal data was derived for each ETDRS region as explained elsewhere.^{41 ,42} (link) In short, the median of each of the three measurements per day and its difference to the overall mean across the 3 days were calculated. Subsequently, the resulting deltas were evaluated in a cumulative distribution function. The 95% limits of the cumulative distribution function were determined and multiplied by $\sqrt{3/2}$ as a correction factor for centered data. The CR was considered half the length of the 95% interval. It is noteworthy that only intersession but not intrasession repeatability was considered in the analysis, to reflect most studies that investigate choroidal metrics in an intersession rather than intrasession (consecutive) manner.